In this world there are those who have great power; to be stronger, faster, and basically invulnerable to damage by anyone but each other. The trick is, though, that they cannot remain in this state for a great deal of time, and must spend the vast majority of their time in mortal form. So, most of the time they are just as vulnerable and weak as the rest of us, but then they can switch for a short while into being superhuman.

The mechanics would likely involve two sets of character sheets. One in which they're mostly average at everything, and another where they're crazy powerful and have stunts and the like.

I'm thinking that stress would not carry between states, but consequences would.

Then, when they've been in super form for a period of time, they begin to take stress to represent the toll it's taking on them.

Maybe it'd be the case where every turn they make a roll to overcome "Stability" or something, and if they fail they take stress, and the longer it's been, the harder the difficulty to overcome.