Just do whatever you want, as long as there's a counter to it, or if they give up a lot of their powers to gain a different set. For example:
My character, Jay, is unable to use magic of any kind, and anything that he touches that's magical breaks or fizzles. This means that he's often limited to melee combat, but is otherwise unstoppable against mages. But he also has a gauntlet that turns his disease into a mana absorption, which it converts into magic he can use, which is how he can be a summoner. While wearing it, though, he's susceptible to a multiple personality disorder that can be very chaotic sometimes and he's also susceptible to powerful magic spells.
See? Not OP, but not UP, either.