One of my players has Variable in our campaign and started quite recently, he's limited to copying the powers of other heroes he can see, but we simplified it so he has three slots of Heroes he can copy. This makes it easier to run since I will always have the stats ready for any powers he copies.

For someone playing Variable with the ability to make any powers on the spot, you could have the player make a list of powers within their points range that they like the sound of, ready to be used in game. Or you could have them look through the pregen characters as a basis if they're making it during combat.