In addition to superhero comic books themselves, I will often look at some of the pen & paper role-playing game books that I have (some quite old) to see what hasn't already been added to City of Heroes: Rebirth, to see if they could conceivably work in City.
To start this off, I'm looking at the Mutants & Masterminds RPG, 2002 edition from Green Ronin Publishing, which used a variant of the D20 system rules started by Dungeons & Dragons 3rd edition. They had a Shapeshift power that I thought might be a starting point for a Mutant Epic AT (non-aligned versus Hero Kheldians and Villain Arachnos Soldiers/Widows). The actual change of appearance ability I thought could be handled as a type of Stealth/Invisibility/Hide power, where the character activates one power that gives them the appearance of a given enemy unit, similar to existing Halloween costume powers. On activation of any aggro'ing power, the character's appearance reverts to normal and they generate aggro as normal. Establishing or resetting the power would require selecting an active or defeated enemy and then activating - I would probably want to see the toggle suppression being developed for Kheldians to work here, rather than necessarily trying have someone figure out how to get all of the player power effects to work on all enemy models. Personally, I don't think this aspect is TOO far out of bounds, although whether a given disguise would work against all enemy groups within a given area might bump into suspension of disbelief issues (i.e., I wouldn't want to force a player to constantly reset their Disguise power(s) whenever they meet another enemy group on a given map).
The M&M Shapeshift power also has "extras", other types of powers that can be added onto a given power under a single point cost. One of the "extras" for Shapeshift is Mimic, which allows the character to copy and use an enemy's specific powers. I think this would be an absolute nightmare to code, so I'm not really even wanting to bring it up, but I wanted to make sure you knew that I knew about the concern.
I've got more after this, but I figured to get this started here.