So about six minutes ago I had this idea for someone whose power is sort of duplication, but each duplicate represents a different part of the character's personality...and who has different powers. She or he splits and the new character represents something else. Sometimes the character pulls out the butt-kicking side, sometimes the brainy side, sometimes the manipulative emotion-controlling side, and sometimes the transformative "I'll change to be with you" chameleon side.
This is not a power that I immediately said, "Hey, it's easy to do."
This is kind of like Alter Ego, in that it's a different set of powers and a different character, and kind of like Duplication, in that both versions are around. (Otherwise, it would be straight Alter Ego.)
But it suddenly occurred to me that you could do it like this:
Duplication, Extra: Side Effect (Alter Ego)
Every time the duplication goes off, boom, the Alter Ego goes off for the duplicate. And you can either use a straight there-are-n-duplicates (except when narratively interesting) or you can take the Dial-H-for-Hero serial extra on Alter Ego, and the character never knows what duplicate he's going to get, whether it's a known duplicate or some other aspect of personality.
As a GM, I'd probably ask the player to not go with the serial Alter Ego duplicate, just to keep things faster at the table.
EDIT the first: This is, of course, the Crazy Jane/Black Annis power, which I only just realized.
EDIT the second: Or, as pointed out by the lovely and talented Fabricio, you can do serial Alter Ego power by having the player or the GM generate a series of heroes that the character will turn into. Depending on the degree of trust between GM and player, the GM might want to look them over before the game, but they're all likely to be gone soon. (There is the plotline where one of the supercharacter refuses to turn back--the Miracleman Jr. scenario--but the player and the GM will probably agree on that, because it isn't the concept of the character.)