I'm under the impression that they have the right to use whatever the users of the game (aka us) make for it (at least that's what I think it says on the very first screens when you turn the game on).
Second, if I were a programmer, I'd definitely check the mods, and grab the better ideas, at least as a starting point.
Third, Iron Fist is the only 100% original mod character that's going to be in MUA2. From the rest of the roster:
a) they were originally in MUA1, either out of the box or DLC.
b) their mods were based on their XML appearances (Juggernaut, Gambit, Jean Grey)
c) their mods don't exist (Green Goblin, Songbird, Penance)
so there isn't a lot they could've copied.