There are a lot of ways to take it. If you want the transforming piece, go with something like magical amulet. There is certainly a lot of magic in the Marvel Universe. Heck i know of a character in marvel (minor character) who pretty much hulks out and loses control of himself if he doesnt have the amulet.
You could always do something along the lines of an amulet, but like a drawback could be something like the longer you use it in a go the less control you have over yourself, to the point where eventually someone might have to knock you out to keep it under control. You could have a curse or something to keep it in your possession or bad things happen, or any number of things. If you want the transforming character, just work with it and find a way to make it work.
It seems like the goal here is to keep characters fairly grounded in humanity, and to keep them grounded as far more flawed and less than godlike. Remember Marvel is more humans trying to deal with god problems.