this is an interesting question though. It is highly unlikely tobi is madara, as minato stated in recent episodes. Izuna is a possibility, but I doubt it. One thing I found interesting was when minato hit tobi with the rasengan his hand sort of slopped off similar to orochimoru. there has to be some connection. I'd like to throw in the possibility that tobi is in fact sasuke and itachi's father. Just before the nine tails attack we saw itachi holding infant sasuke and wondering why both parents went out on that night. I just found it quite interesting that scene was there and why itachi said what he said. tobi could be obito if someone had used impure resurrection to revive him with no control device,