Just to note Mask Presence's defense is supressed as soon as you come out of stealth so you're only going to get about 3% bonus during fights.
I personally would go for more +regen as you don't have any form of healing apart from green insps. I'd also drop CT:O as you have quite a decent amount of +tohit and +acc as is.
Imo ML is overslotted, maybe 4 slot it and dump in 2x Adjusted Targeting (rech & tohit/rech/end), and def/rech lotg with the +rech.
Here's my widow's i19 build you can use as a comparison:
Villain Plan by Mids' Villain Designer 1.81
http://www.cohplanner.com/
Click this DataLink to open the build!
Misscene: Level 50 Natural Arachnos Widow
Primary Power Set: Night Widow Training
Secondary Power Set: Widow Teamwork
Power Pool: Flight
Power Pool: Speed
Power Pool: Leaping
Ancillary Pool: Soul Mastery
Villain Profile:
Level 1: Swipe -- T'Death-Acc/Dmg(A), T'Death-Dmg/EndRdx(3), T'Death-Dmg/Rchg(3), T'Death-Acc/Dmg/EndRdx(5), T'Death-Dmg/EndRdx/Rchg(5), T'Death-Dam%(7)
Level 1: Combat Training: Defensive -- LkGmblr-Def(A), LkGmblr-Rchg+(7)
Level 2: Strike -- Mako-Acc/Dmg(A), Mako-Dmg/EndRdx(9), Mako-Dmg/Rchg(9), Mako-Acc/EndRdx/Rchg(11), Mako-Acc/Dmg/EndRdx/Rchg(11), Mako-Dam%(13)
Level 4: Tactical Training: Maneuvers -- RedFtn-Def/EndRdx(A), RedFtn-Def/Rchg(13), RedFtn-Def/EndRdx/Rchg(15), RedFtn-Def(15), RedFtn-EndRdx(17), RedFtn-EndRdx/Rchg(17)
Level 6: Hover -- Zephyr-Travel(A), Zephyr-Travel/EndRdx(19), LkGmblr-Rchg+(27)
Level 8: Follow Up -- C'ngImp-Acc/Dmg(A), C'ngImp-Dmg/Rchg(19), C'ngImp-Acc/Dmg/Rchg(21), C'ngImp-Acc/Dmg/EndRdx(21), C'ngImp-Dmg/EndRdx/Rchg(23)
Level 10: Hasten -- RechRdx-I(A), RechRdx-I(29), RechRdx-I(29)
Level 12: Indomitable Will -- S'fstPrt-ResDam/Def+(A)
Level 14: Fly -- Zephyr-Travel(A), Zephyr-Travel/EndRdx(23)
Level 16: Lunge -- Hectmb-Dmg/Rchg(A), Hectmb-Acc/Dmg/Rchg(45), Hectmb-Acc/Rchg(45), Hectmb-Dmg/EndRdx(45), Hectmb-Dam%(46)
Level 18: Spin -- Armgdn-Dmg/Rchg(A), Armgdn-Acc/Dmg/Rchg(40), Armgdn-Acc/Rchg(40), Armgdn-Dmg/EndRdx(42), Armgdn-Dam%(42)
Level 20: Confront -- Taunt-I(A)
Level 22: Slash -- Mako-Acc/Dmg(A), Mako-Dmg/EndRdx(33), Mako-Dmg/Rchg(34), Mako-Acc/EndRdx/Rchg(34), Mako-Acc/Dmg/EndRdx/Rchg(34), Mako-Dam%(36)
Level 24: Mask Presence -- RedFtn-Def/EndRdx(A), RedFtn-Def/Rchg(31), RedFtn-EndRdx/Rchg(42), RedFtn-Def/EndRdx/Rchg(43), RedFtn-Def(50), RedFtn-EndRdx(50)
Level 26: Mental Training -- Flight-I(A)
Level 28: Tactical Training: Leadership -- AdjTgt-ToHit(A), AdjTgt-ToHit/Rchg(36), AdjTgt-ToHit/EndRdx/Rchg(36), AdjTgt-EndRdx/Rchg(37), AdjTgt-ToHit/EndRdx(37)
Level 30: Mind Link -- LkGmblr-Def/Rchg(A), LkGmblr-Rchg+(37), AdjTgt-ToHit/Rchg(39), AdjTgt-EndRdx/Rchg(43)
Level 32: Tactical Training: Assault -- EndRdx-I(A)
Level 35: Foresight -- RedFtn-Def/EndRdx(A), RedFtn-Def/Rchg(39), RedFtn-EndRdx/Rchg(39), RedFtn-Def/EndRdx/Rchg(40), RedFtn-Def(43)
Level 38: Placate -- RechRdx-I(A)
Level 41: Gloom -- Decim-Acc/Dmg(A), Decim-Dmg/EndRdx(46), Decim-Dmg/Rchg(46), Decim-Acc/EndRdx/Rchg(48), Decim-Acc/Dmg/Rchg(48)
Level 44: Combat Jumping -- LkGmblr-Def/EndRdx(A), LkGmblr-Rchg+(50)
Level 47: Tactical Training: Vengeance -- LkGmblr-Def/Rchg(A), LkGmblr-Rchg+(48)
Level 49: Elude -- RechRdx-I(A)
------------
Level 1: Brawl -- Empty(A)
Level 1: Sprint -- ULeap-Stlth(A)
Level 2: Rest -- Empty(A)
Level 1: Conditioning
Level 4: Ninja Run
Level 1: Swift -- Flight-I(A)
Level 1: Hurdle -- Jump-I(A)
Level 1: Health -- Numna-Heal(A), Numna-Regen/Rcvry+(25), Mrcl-Heal(25), Mrcl-Rcvry+(27)
Level 1: Stamina -- P'Shift-EndMod(A), P'Shift-EndMod/Rchg(31), P'Shift-EndMod/Acc/Rchg(31), P'Shift-EndMod/Acc(33), P'Shift-End%(33)
------------
Code:
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http://www.scene-and-heard.com/cov/covsig.jpg
Heisenberg, I'm (slowly) leveling up a NW, and have him to 42 now. I think I'll try your build for sure, as it is pretty darn good and saves me a few billion (that's 'illion' with a capitol B) inf and as much as I like market PvP, I'd rather save the money. I'll hopefully be back here to report some time before too long.
@Quantum Evil Rad/Rad Corruptor
Making the world safe for maleficent particles since 2004.
One reason to consider keeping CT:O is for the toHit debuff resistance. For debuffs that don't have to roll toHit against you, having a way to resist them is very nice. Many such debuffs are pretty strong, so having some resist helps a Widow's frequently high +toHit better overcome them. I'm actually planning to add it to my build as a one-slot wonder in I19.
Blue
American Steele: 50 BS/Inv
Nightfall: 50 DDD
Sable Slayer: 50 DM/Rgn
Fortune's Shadow: 50 Dark/Psi
WinterStrike: 47 Ice/Dev
Quantum Well: 43 Inv/EM
Twilit Destiny: 43 MA/DA
Red
Shadowslip: 50 DDC
Final Rest: 50 MA/Rgn
Abyssal Frost: 50 Ice/Dark
Golden Ember: 50 SM/FA
I suggest Dart Burst over Psychic Scream. The range is slightly less but the damage is higher, the activation time is lower, the debuff is better and it recharges in half the time. With the recharge you are sporting you could Dart Burst, jump in, Spin, jump out, Dart Burst as much as you like.
Also you are seriously hampering yourself by slotting Follow Up with full Gaussian. You don't really need the 6 piece bonus since your defenses already exceed the cap and you have defense debuff resistance as well. I suggest 4 of Mako's Bite, the Gaussian Proc and a Damage Proc.
Here's my planned i19 Night Widow build, seeing as others are posting theirs. 
Mine makes use of the 60 month vet badge to add Fly at level 6. I didn't take Hasten, as Mind Link is already perma without it (just under five second overlap time, enough to recast, and build has 60% recharge debuff resistance). I took Elude for emergency defense debuff cascade and when I'm bored and feel like running instead of flying, haha. Build doesn't make use of purples, but it does have three HOs.
Reserved slots are full set of Performance Shifter (for the AoE bonus), and Numina and Miracle procs, and either one of the single Heals from the two, or a Regen Tissue one.
Villain Plan by Mids' Villain Designer 1.81
http://www.cohplanner.com/
Click this DataLink to open the build!
Operative Acarina NW i19: Level 50 Natural Arachnos Widow
Primary Power Set: Night Widow Training
Secondary Power Set: Widow Teamwork
Power Pool: Flight
Power Pool: Medicine
Villain Profile:
Level 1: Poison Dart
- (A) Thunderstrike - Accuracy/Damage
- (3) Thunderstrike - Damage/Endurance
- (5) Thunderstrike - Damage/Recharge
- (7) Thunderstrike - Accuracy/Damage/Recharge
- (11) Thunderstrike - Accuracy/Damage/Endurance
- (33) Thunderstrike - Damage/Endurance/Recharge
- (A) Luck of the Gambler - Recharge Speed
- (45) Gift of the Ancients - Run Speed +7.5%
- (A) Mako's Bite - Accuracy/Damage
- (3) Mako's Bite - Damage/Endurance
- (5) Mako's Bite - Damage/Recharge
- (7) Mako's Bite - Accuracy/Endurance/Recharge
- (29) Mako's Bite - Accuracy/Damage/Endurance/Recharge
- (36) Mako's Bite - Chance of Damage(Lethal)
- (A) Luck of the Gambler - Recharge Speed
- (39) Luck of the Gambler - Defense
- (39) Luck of the Gambler - Defense/Endurance
- (40) Luck of the Gambler - Defense/Endurance/Recharge
- (A) Blessing of the Zephyr - Run Speed, Jump, Flight Speed, Range
- (48) Blessing of the Zephyr - Run Speed, Jump, Flight Speed, Range/Endurance
- (50) Winter's Gift - Slow Resistance (20%)
- (A) Mako's Bite - Accuracy/Damage
- (9) Mako's Bite - Damage/Recharge
- (9) Mako's Bite - Accuracy/Endurance/Recharge
- (17) Mako's Bite - Accuracy/Damage/Endurance/Recharge
- (34) Gaussian's Synchronized Fire-Control - Chance for Build Up
- (50) Gaussian's Synchronized Fire-Control - To Hit Buff/Recharge
- (A) Steadfast Protection - Resistance/Endurance
- (11) Steadfast Protection - Resistance/+Def 3%
- (A) Crushing Impact - Accuracy/Damage
- (13) Crushing Impact - Damage/Endurance
- (13) Crushing Impact - Accuracy/Damage/Recharge
- (29) Crushing Impact - Accuracy/Damage/Endurance
- (34) Crushing Impact - Damage/Endurance/Recharge
- (A) Scirocco's Dervish - Accuracy/Damage
- (15) Scirocco's Dervish - Damage/Endurance
- (15) Scirocco's Dervish - Damage/Recharge
- (33) Scirocco's Dervish - Accuracy/Recharge
- (33) Scirocco's Dervish - Accuracy/Damage/Endurance
- (A) Endurance Reduction IO
- (A) Crushing Impact - Accuracy/Damage
- (19) Crushing Impact - Damage/Endurance
- (19) Crushing Impact - Accuracy/Damage/Recharge
- (23) Crushing Impact - Accuracy/Damage/Endurance
- (31) Crushing Impact - Damage/Endurance/Recharge
- (A) Luck of the Gambler - Recharge Speed
- (37) Red Fortune - Defense/Endurance
- (A) Run Speed IO
- (A) Luck of the Gambler - Recharge Speed
- (25) Luck of the Gambler - Endurance/Recharge
- (25) Luck of the Gambler - Defense/Recharge
- (39) Red Fortune - Defense/Recharge
- (40) HamiO:Membrane Exposure
- (A) Scirocco's Dervish - Accuracy/Damage
- (27) Scirocco's Dervish - Damage/Endurance
- (27) Scirocco's Dervish - Damage/Recharge
- (31) Scirocco's Dervish - Accuracy/Recharge
- (31) Scirocco's Dervish - Accuracy/Damage/Endurance
- (A) Luck of the Gambler - Recharge Speed
- (37) Luck of the Gambler - Defense
- (45) Luck of the Gambler - Defense/Recharge
- (48) Gift of the Ancients - Run Speed +7.5%
- (A) Recharge Reduction IO
- (46) Recharge Reduction IO
- (A) Dark Watcher's Despair - To Hit Debuff/Recharge
- (34) Dark Watcher's Despair - To Hit Debuff/Recharge/Endurance
- (36) Dark Watcher's Despair - To Hit Debuff/Endurance
- (36) Dark Watcher's Despair - Recharge/Endurance
- (A) Healing IO
- (A) Doctored Wounds - Heal/Endurance
- (40) Doctored Wounds - Heal/Recharge
- (43) Doctored Wounds - Heal/Endurance/Recharge
- (45) Doctored Wounds - Heal
- (46) Doctored Wounds - Recharge
- (46) Interrupt Reduction IO
- (A) Positron's Blast - Accuracy/Damage
- (42) Positron's Blast - Damage/Endurance
- (42) Positron's Blast - Damage/Recharge
- (42) Positron's Blast - Damage/Range
- (43) Positron's Blast - Accuracy/Damage/Endurance
- (43) Positron's Blast - Chance of Damage(Energy)
- (A) HamiO:Cytoskeleton Exposure
- (A) Recharge Reduction IO
- (A) HamiO:Membrane Exposure
Level 1: Brawl
- (A) Empty
- (A) Empty
- (37) Empty
- (48) Empty
- (A) Empty
- (17) Empty
- (21) Empty
- (21) Empty
- (23) Empty
- (50) Empty
Level 4: Ninja Run
------------
Ideon's Paragonwiki page
Member of Paragon/Rogue Knights
Arc: 60092 - Supa Rumble in the Park
"Keep living the dream, and never let any jerk tell you what to do."
-- High-Roller
And Ideon swoops in with the build jam packed with the most Win.
Why would you slot sprint and rest, and also not put anything in there? I would figure to 6 slot the high level attacks, not the low level ones, and especially not sprint and rest... Am I missing something? 
With inherent Fitness Pool just around the corner I thought I'd go re-visit an old Night Widow fantasy build and see what I could do with it. My goal with the build was to get perma-Hasten as well as soft-capped defenses. The old build had 3 purple sets in it though, so I figured I'd see if I could trim that down. This is the result, with no purples at all! It's still an expensive build but should be much more reasonable now. Any and all criticism is welcome since frankly it's been a while since I've played around with builds, or played a Widow for that matter, so any glaring mistakes I made I'd appreciate having pointed out to me.
Villain Plan by Mids' Villain Designer 1.707
http://www.cohplanner.com/
Click this DataLink to open the build!
Night Widow: Level 50 Natural Arachnos Widow
Primary Power Set: Night Widow Training
Secondary Power Set: Widow Teamwork
Power Pool: Speed
Power Pool: Leaping
Ancillary Pool: Mu Mastery
Villain Profile:
Level 1: Swipe
- (A) Crushing Impact - Accuracy/Damage
- (3) Crushing Impact - Damage/Endurance
- (5) Crushing Impact - Damage/Recharge
- (7) Crushing Impact - Accuracy/Damage/Recharge
- (36) Crushing Impact - Accuracy/Damage/Endurance
- (36) Crushing Impact - Damage/Endurance/Recharge
Level 1: Combat Training: Defensive- (A) Luck of the Gambler - Recharge Speed
Level 2: Strike- (A) Crushing Impact - Accuracy/Damage
- (3) Crushing Impact - Damage/Endurance
- (5) Crushing Impact - Damage/Recharge
- (7) Crushing Impact - Accuracy/Damage/Recharge
- (25) Crushing Impact - Accuracy/Damage/Endurance
- (36) Crushing Impact - Damage/Endurance/Recharge
Level 4: Combat Training: Offensive- (A) Accuracy IO
Level 6: Tactical Training: Maneuvers- (A) Luck of the Gambler - Defense/Endurance
- (11) Luck of the Gambler - Defense
- (21) Luck of the Gambler - Recharge Speed
- (45) Luck of the Gambler - Defense/Recharge
Level 8: Follow Up- (A) Gaussian's Synchronized Fire-Control - To Hit Buff
- (9) Gaussian's Synchronized Fire-Control - To Hit Buff/Recharge
- (9) Gaussian's Synchronized Fire-Control - To Hit Buff/Recharge/Endurance
- (11) Gaussian's Synchronized Fire-Control - Recharge/Endurance
- (34) Gaussian's Synchronized Fire-Control - To Hit Buff/Endurance
- (37) Gaussian's Synchronized Fire-Control - Chance for Build Up
Level 10: Indomitable Will- (A) Steadfast Protection - Resistance/+Def 3%
- (34) Impervium Armor - Resistance/Endurance
- (45) Impervium Armor - Resistance/Endurance/Recharge
Level 12: Lunge- (A) Crushing Impact - Accuracy/Damage
- (13) Crushing Impact - Damage/Endurance
- (13) Crushing Impact - Damage/Recharge
- (17) Crushing Impact - Accuracy/Damage/Recharge
- (34) Crushing Impact - Accuracy/Damage/Endurance
- (37) Crushing Impact - Damage/Endurance/Recharge
Level 14: Spin- (A) Obliteration - Damage
- (15) Obliteration - Accuracy/Recharge
- (15) Obliteration - Damage/Recharge
- (17) Obliteration - Accuracy/Damage/Recharge
- (37) Obliteration - Accuracy/Damage/Endurance/Recharge
Level 16: Tactical Training: Assault- (A) Endurance Reduction IO
Level 18: Slash- (A) Mako's Bite - Accuracy/Damage
- (19) Mako's Bite - Damage/Endurance
- (19) Mako's Bite - Damage/Recharge
- (21) Mako's Bite - Accuracy/Endurance/Recharge
- (25) Mako's Bite - Accuracy/Damage/Endurance/Recharge
Level 20: Hasten- (A) Recharge Reduction IO
- (23) Recharge Reduction IO
- (33) Recharge Reduction IO
Level 22: Foresight- (A) Luck of the Gambler - Defense
- (23) Luck of the Gambler - Recharge Speed
Level 24: Mental Training- (A) Run Speed IO
Level 26: Eviscerate- (A) Obliteration - Damage
- (27) Obliteration - Accuracy/Recharge
- (27) Obliteration - Damage/Recharge
- (33) Obliteration - Accuracy/Damage/Recharge
- (33) Obliteration - Accuracy/Damage/Endurance/Recharge
Level 28: Mind Link- (A) Kismet - Defense/Endurance
- (29) Kismet - Defense/Recharge
- (29) Kismet - Endurance/Recharge
- (31) Kismet - Defense/Endurance/Recharge
- (39) Kismet - Accuracy +6%
- (46) Luck of the Gambler - Recharge Speed
Level 30: Mask Presence- (A) Luck of the Gambler - Recharge Speed
Level 32: Psychic Scream- (A) Positron's Blast - Accuracy/Damage
- (40) Positron's Blast - Damage/Endurance
- (40) Positron's Blast - Damage/Recharge
- (40) Positron's Blast - Damage/Range
- (42) Positron's Blast - Accuracy/Damage/Endurance
Level 35: Tactical Training: Leadership- (A) Adjusted Targeting - To Hit Buff
- (42) Adjusted Targeting - To Hit Buff/Endurance
- (45) Adjusted Targeting - To Hit Buff/Endurance/Recharge
- (46) Adjusted Targeting - Endurance/Recharge
Level 38: Elude- (A) Recharge Reduction IO
- (39) Recharge Reduction IO
- (39) Recharge Reduction IO
Level 41: Electrifying Fences- (A) Positron's Blast - Accuracy/Damage
- (42) Positron's Blast - Damage/Endurance
- (43) Positron's Blast - Damage/Recharge
- (43) Positron's Blast - Damage/Range
- (43) Positron's Blast - Accuracy/Damage/Endurance
Level 44: Mu Lightning- (A) Decimation - Accuracy/Damage
- (46) Decimation - Damage/Endurance
- (50) Decimation - Damage/Recharge
- (50) Decimation - Accuracy/Endurance/Recharge
- (50) Decimation - Accuracy/Damage/Recharge
Level 47: Summon Striker- (A) Call to Arms - Endurance/Damage/Recharge
- (48) Call to Arms - Accuracy/Damage/Recharge
- (48) Call to Arms - Accuracy/Damage
- (48) Call to Arms - Accuracy/Recharge
Level 49: Super Jump- (A) Jumping IO
------------Level 1: Brawl
- (A) Empty
Level 1: Sprint- (A) Empty
Level 2: Rest- (A) Empty
Level 1: ConditioningLevel 2: Ninja Run
Level 1: Swift
- (A) Run Speed IO
Level 1: Hurdle- (A) Jumping IO
Level 1: Health- (A) Miracle - +Recovery
Level 1: Stamina- (A) Performance Shifter - Chance for +End
- (31) Performance Shifter - EndMod
- (31) Endurance Modification IO
------------