Immaterial shoudl always stay immaterial. That's why i dislike un-immaterialing creatures. A creature that has the same effect as a PU just sounds like an interesting concept. That way, you get a useful card while in play, and when you could play a PU, you already have a card that can turn into that creature. It's like a PU but a bit more powerful, yet not making PU obsolete, since not every deck with PU uses sparks.