most have two attacks, one boost, and an xtreme, no passives taht i know of, mostly cause i never understood how they worked. The labels used by these characters to get there powers to show up in the dangeroom are "power_attack", "power_smash", "power_boost" and "power_xtreme".
But not all of them only have four powers, both Pyro and Magneto have more than that, but the dangeroom and npc that you can control only allow four powers, during battle they use all of them against you. Oh i also forgot you can also have "attacklight1" or "attackheavy1", which can be a cheat to add more powers.
i dont really know about flasback missions, im trying to figure out how to write a script, like the npc 27 mua project, so that we can use any of these characters in game just by going to visit prof x or someone liek that. good luck, it sounds like you have a cool idea.