Assassination/Hits
Not too bad.
1) No AV's... EVER. They bore me to tears when I'm fighting them while teamed.
2) Easiest set up with their "ambush" spawn code, which shows some of the limitations. Just like ambushes, the players travel too fast. NPC spawns have trouble tracking and keeping up. Most players never "encounter" the foe... still, if these guys could spawn in mission instances...

A suggesstion
I agree with Chase AV's should not be part of the assassination squad. My reasons are different I think getting ambushed by an AV when you are in the middle of something else could be nasty, also they are a trump card not to be wasted.
If anything you could make the employer an AV, so that when they get enough clues they get an radio mission for an AV battle, I feel that the Emplayer AV battle should be the climax of this not the assassin squads.
Slikill's idea about the employer/contractor being the climax is dead-on.
Another reason to NEVER let an ambush include an AV (this also applies to EBs) is aggro-splash. Just because I happen to hit my trigger-number at the end of a lvl 45 mish doesn't mean I'm not in Steel Canyon at the time. A lot of bystanders could be easily ganked from the AV/EB AOE if not from just getting in his way.
--If we can have huge sig images, why can we have only five lines of text?
--...faceplanting like a Defender pulling an AV (Nalrok_AthZim)
Edited OP.
About not having AV assassins. I was thinking the same about not having them for the high level players, but I wasn't sure. So leiutenants after low level players 1-14, bosses after mid level player 15-35, and EBs after 36-50. The assassins level matters to the player difficulty -2 for heroic/villainous to +2 for invincible/something, what ever it is villains have.
With the ambush problem about the player getting away too easily. The assassin could use the code that it follows the player where ever they go like in Mayhem missions. There could be a count down timer for how long the player can stay away from the assassin, when the assassin gives up and stops persuing the player.
The assassin could be designed to use only single target powers since their target most of the time is a single player. That way player who want to mind their business and not get involved won't have to.
For the employer suggested as an AV alright but mainly for a team for 5+ under 5 the mission is against an EB.
If it ain't broke set it on fire, then say it was a fault in the design.
Main:50 Force Encephalon Mind/Kinetic Controller, Protector Server
If a player defeats X NPCs in a group in X amount of time. That group sends an assassin/hitman to take out the player.
Only NPCs the player defeats in a group adds toward the counter for that group.
Getting a Mission Complete gives a bonus to the counter to the group the mission was against.
Multiple Groups can send hits after a player. But only one assassination/hit can be made at a time. Between 10 minutes to 40 minutes after the last assassin/hitman stops chasing the player. No less than 30 seconds to 40 minutes after a player leaves a mission.
The counter discharges slow enough to players with slow killing builds if they work only on angering NPC groups at a skim of 3. This way any player can use this with only moderate effort. And it discharges fast enough so only a few groups will go after fast killing players.
When the player is being targeted a flash message will read, "YOU ARE BEING TARGETED".
Low level players will have a lieutenant sent after them, mid level players will have a boss sent after them, and high level players will have an EB sent after them.
Assassin's/Hitman's level depends on the players difficulty. -2 for heroic/villainous and +2 for invincible/something, whatever villains use.
Assassins'/Hitmen's primarly goes after the player tagged for the hit. They have taunt protection on for 10 seconds and off for 4; while it's on they will go only after their target. They won't attack anyone other the target that doesn't attack them.
Assassins/Hitmen use only single target attacks to avoid hitting player that don't want to get involved.
Assassins/Hitmen won't attack near kill zones.
Assassins/Hitmen will persue a player everywhere, like how Mayhem missions work. If the player can get to a safe zone or stay out of range of the assassin's/hitman's power for 30 seconds. The assassin/hitman stops persuing the player and head for the nearest door.
If the assassin/hitman stops chasing the player a flash message will appear, "YOU ARE NO LONGER BEING PERSUED"
Assassins will only have travel powers if the player has travel powers.
Assassins/Hitmen will have buffs and use debuffs depending on the players build. The buffs and debuffs aren't to make the assassins/hitmen too hard only challenging.
Defeating assassins/hitmen earns a clue. A hit receit with the employer's name. The player could choose to go after the employer. The employer on a team of 5 or more would spawn an AV, teams of 2 to 4 would spawn an EB. And soloing the boss would be a boss designed to be buffed against the player's character.
Logging out will pause the counter.
Assassins/Hitmen won't go after a player until 10 minutes after they've logged in.
If it ain't broke set it on fire, then say it was a fault in the design.
Main:50 Force Encephalon Mind/Kinetic Controller, Protector Server