I've found Oppressive Gloom to be the jewel of all the Defender APP's, especially with a single target stun to stack with it. It requires next to no slotting, and has negligible end cost. The HP drain is barely noticeable.
I'd suggest taking Oppresive Gloom, Scramble Thoughts, Soul Drain and Dark Embrace as your last four picks. Its a more melee-centric style of play maybe, but you're probably tough enough to survive in there with 24% Defence and a mez aura, and PFF if things start to go bad.
I believe that every Defender should take Assault. At least until the devs look at Defenders in general once again or until we get access to heroic Corrupters.
I'm always a fan of melee-centric Defenders, but, man, I've been one-shotted sooooooooooo many times. I've had my Psy Scream tag a corner mob, outside the tank's range and BAM--I'm gone. It's probably better with Mind Over Body, but 24% Def doesn't feel like that much.
I would be happier with Oppressive Gloom if I had another AOE stun like Thunderclap or Stalagmites rather than a single target. Or are you saying that OG is powerful enough on its own?
I also wanted Assault, but I took Maneuvers to stretch that Def; and dipped into Aid Other so I could take Aid Self. When low on HP, PFF + AS and pop out--usually with Repulsion on. It's such a safety net (and FF is all about safety) that I thought it was better. Am I wrong?
There is no "right" or "wrong" in how to build and play a character... it's all about what is comfortable and more FUN for you!
Taking the Medicine pool makes a lot of sense with Force Fields, and is a popular choice
Aid Self and PFF work really well together.
Aid Other also allows you to patch up teammates as they leak HP very slowly, and your chances of being interrupted are reduced by your high-ish defence.
Oppressive Gloom on its own will stun minions, and will stun bosses when stacked with a Single target stun like Scramble Thoughts. It allows you to walk up to a boss and stun them before they can react - I used to do this with Screech and OG on my FF/Sonic and found it really effective.
I prefer solo and small teams, maybe this tactic works better there than on 8 person groups.
I wouldn't worry about Assault myself. Its a nice bonus, sure, but numerically only ups team damage by 10% or so. If you really want to do this, the Sonic Blast secondary gives you Howl, which will increase team damage by 20% due to the -Res, with more -res to stack on hard targets. I'm not saying Assault is worthless, just putting it into perspective.
BTW - your forum handle rocks, Secret Asian Man! 
Yes, yes, fun all around. I'm having a good time with my FFer, I'm just trying to explore what other tricks she may have. Basically, I can solo nearly any rescue mission and hold my own on teams, contributing with high Def and some -Recharge, but I was looking for other options that people enjoyed. I'm eager to try, but only have so many free respecs to nudge just a few powers.
Stunning sounds like great damage control, but I read elsewhere about OG's low magnitude. If I could mass stun a group, that'd be great, but stunning one at a time seems less productive.
I guess I was hoping for an ace in the hole that I hadn't discovered yet.
Thanks for the responses!
Jeez. I just realize two people gave me advice--and I dismissed it without trying. Sorry guys.
Right on, my next repecc, I'll try Dark with OG and Scramble Thoughts and report back.
If you take one of the resistance based armors in the APPs, then you can get the Steadfast Protection +3% Defense IO to stack with Dispersion Bubble. My FF/Energy just hit 50 and I went with Assault, Maneuvers, Aid Other and Aid Self from the power pools. From the APP, I took Electric Fence and Charged Armor, mostly as placeholders for IO's. With IO's its pretty easy to cap your ranged defense and my melee and AoE defenses are nearly capped too.
So my Force Fielder is at level 41 now and it's been fun. This is my team build below and my bubbles provide 44.46% defense and my personal defense at about 24.1% (with the realization that no Defender APP has any defense based shields).
I had enough toggles with Hover, Dispersion Shield, Repulsion Field (only when swarmed), Maneuvers, and Tactics that I decided an Endurance draining Nuke was a bad idea, so I skipped Psychic Wail. And without another stun to stack on the single target Scramble Thoughts, I skipped that as well. And I prefer the precision of Repulsion Field to Force Bubble.
As I moved into the Defender APPs, I was feeling really meh. I didn't take three of my high tier powers and the pools looked a little lack luster. I took Psi for the theme, but only have Mass Hypnosis. I dropped Tactics because the endurance-sucking Telekinesis was coming up--thinking the mass hold was more useful than the accuracy; and wanted Dominate to stack on bosses.
I found that I could Psychic Scream, Psychic Tornado, Repulsion Bomb and then Mass Hypnosis mobs into submission, picking off the remainders with my single targets. It's okay fun, but am I missing out by not going another route? Was there some kind of awesome-ness that wasn't obvious in the other APPs?
Thanks in advanced!
Hero Plan by Mids' Hero Designer 1.601
http://www.cohplanner.com/
Click this DataLink to open the build!
Marvel Ghoul: Level 50 Mutation Defender
Primary Power Set: Force Field
Secondary Power Set: Psychic Blast
Power Pool: Fitness
Power Pool: Flight
Power Pool: Leadership
Power Pool: Medicine
Ancillary Pool: Psychic Mastery
Hero Profile:
Level 1: Personal Force Field -- RechRdx(A)
Level 1: Mental Blast -- Acc(A), Acc(43), Dmg(46), Dmg(46), RechRdx(46)
Level 2: Deflection Shield -- EndRdx(A), DefBuff(3), DefBuff(3), DefBuff(43)
Level 4: Subdue -- Acc(A), Acc(5), Dmg(5), Dmg(9), RechRdx(40), EndRdx(43)
Level 6: Insulation Shield -- EndRdx(A), DefBuff(7), DefBuff(7), DefBuff(9)
Level 8: Swift -- Flight(A)
Level 10: Psychic Scream -- Acc(A), Acc(11), Range(11), Range(37), RechRdx(40), RechRdx(40)
Level 12: Dispersion Bubble -- EndRdx(A), EndRdx(13), DefBuff(13), DefBuff(15), DefBuff(15)
Level 14: Health -- Heal(A)
Level 16: Telekinetic Blast -- Acc(A), Acc(17), Dmg(17), Dmg(36), RechRdx(37), EndRdx(37)
Level 18: Repulsion Field -- EndRdx(A), EndRdx(19), EndRdx(19)
Level 20: Stamina -- EndMod(A), EndMod(21), EndMod(21)
Level 22: Will Domination -- Acc(A), Acc(23), Dmg(23), Dmg(25), RechRdx(25), EndRdx(36)
Level 24: Hover -- DefBuff(A)
Level 26: Repulsion Bomb -- Acc(A), Acc(27), Dmg(27), Dmg(34), RechRdx(34), EndRdx(36)
Level 28: Psionic Tornado -- Acc(A), Acc(29), Dmg(29), Dmg(31), RechRdx(31), EndRdx(31)
Level 30: Fly -- Flight(A)
Level 32: Maneuvers -- EndRdx(A), EndRdx(33), DefBuff(33), DefBuff(33), DefBuff(34)
Level 35: Aid Other -- Heal(A)
Level 38: Aid Self -- Heal(A), Heal(39), RechRdx(39), RechRdx(39)
Level 41: Mass Hypnosis -- Acc(A), Acc(42), Sleep(42), RechRdx(42)
Level 44: Mind Over Body -- EndRdx(A), ResDam(45), ResDam(45), ResDam(45)
Level 47: Dominate -- Acc(A), Acc(48), Hold(48), Hold(48), EndRdx(50)
Level 49: Telekinesis -- EndRdx(A), EndRdx(50), EndRdx(50)
------------
Level 1: Brawl -- Empty(A)
Level 1: Sprint -- Empty(A)
Level 2: Rest -- Empty(A)
Level 1: Vigilance