I'll tell you what I'm looking for in one. something that applies to these 4 criteria (not necessarily in that order):
*originate from the mainstream universe (616, that is)
*is female by birth (sorry, lady loki :()
*has hair showing
*has face (mostly) showing
as I see it, the only ones right now that apply to these criteria are kate bishop and rachel cole. go ahead and throw jane foster into that list if you're willing to compromise for half a face. of course, I'm willing to overlook one of these criteria if the costume in question is good enough. that is something that can't be described officially by criteria, but by something that you just look at and say to yourself "I want that".
I know of one enhanced skin that I want that doesn't apply to all of these rules (or at least just one of these rules) and that's the original nick fury. but I can't imagine him ever making it into the game, considering that marvel now wants to pretend that he never existed in the first place (at least shuma-gorath that have to be alone anymore, heh :D).
there are some that I can think of that could apply to these criteria and make it into the game as enhanced costumes, one of which I think SHOULD make it into the game through the upcoming event, but may or may not because it doesn't really depend on me (or nobody else in the fanbase, for that matter), is clea. and so far there's no indication whether she will actually make it into this event, so I might as well give up all my hopes :(.
anyways, feel free to share what you would like to see in an enhanced skin. I would like to hear what you think.