Character Ideas
Amnesiac: The red room trained more one operative, the Black Widow is gone from the world, but there are others out there waiting to take her place as the most efficient spy/assassin in the world. This character isn't in competition with anyone but undoubtedly stands above the rest, quite nearly undetectable except for the common theme in her ventures. She doesn't remember who she is, and is trying to find out. S.H.E.I.L.D knows part of her past, the fact that she's a red room operative, that what was done to her was likely the same thing that was done to Black Widow, where she can dig up answers and how far the rabbit hole goes. They'll share that information with her if she'll join the new avenging team.
Mutant: A mutant. Not much else besides that. They have a power set that's largely up to you.
Cyborg: Half human half machine, either a S.H.E.I.L.D operative whose life was saved by the program that made them what they are, or a civilian who was kidnapped for a darker purpose who escaped their captors.
Alien: They're not from planet earth, and can do things that other humans can not.