To be clear, I don't know yet. I'm trying to work through this in my mind and hopefully we can figure it out as a team. This will be the MINIMUM required actions to get a new hero working. In my example I'm working on War Machine, since he could be a copy of Iron Man for the sake of figuring things out.
1) A "Stat" file. Since the herostat is no more and is now broken up into individual files under /data/stats/heroes/ each new hero needs their own entry. The menulocation entry doesn't seem to do anything, unlike MUA1. Nanite Nick Fury doesn't even have one in his stat file, and if I am reading this right one location is assigned to more than one character.
2) A talents file
3) PKGB packages, specifically [charactername]_[modelnumber].pkgb and [charactername]_fightstyles.pkgb
4) add entry to /data/stats/heroes/herolisting.xmlb. Add new heroes to the BOTTOM of that list (after Wolverine) or you will get "content missing" errors as apparently it relies on the order in which they're posted.
EDIT: For now I'm going to put this on ice. Unlike MUA1 every character (even DLC) is listed in the .exe file, so we may not be able to add anyone new. I'm going to check model files against the 360 at some point. It'd be really cool to swap out some characters I consider lame with better ones on my 360. No promises, though.