It can't be sage, that would make absolutely no sense. He doesn't even need to do anything he is a GOD to begin with. He died, that's official but there isnt anything said about death of his sons. We know they fought and the younger one won but how did it end?
Just going to drop this bomb here..
Connections between elder son and Tobi :
*believed in peace by power (moon's eye plan goes well in this category)
*got owned by younger son
*Hides his identity
*The way Kabuto and Madara reffered to him in the latest chapters points out that he was someone very strong and powerful
*His techs are beyond anything we have seen in manga
*is arrogant and threats everyone as children
*Tobi wants to become Juubi jinchuurikki, why? To continue his fathers legacy. These elder sons always try to obtain their fathers powers. It's a classic story.
*was talking to Naruto, Kakashi, Yamato about how Senju and Uchiha has always fought and it's their destiny (he witnesses it himself couple of times already)
*is a former shell of himself - either because of being owned by younger son and loosing his powers or because of being extremely old and living thanks to Zetsu
*knows everything there is to know about Rinnegan, Sharingan, SO6P, Uchiha, Senju and everything else
*talks about Madara's fight versus Hashirama as if it was his fight but he's not Madara
This starts to make sense now because Madara could very well have been apprentice of elder son. He was the one pushing Madara to doubt Hashirama and Senju (because of the hate between those clans), he was the one who pushed him to fight Hashirama (to obtain his cells?). He was the one who promised to ressurect Madara after he died. Maybe he even wanted to use Madara to synch with Gedo Mazo and ressurect Juubi after that failed he had an option to either ressurect Madara or to find a new and better host for rinnegan. I think Madara was just to much for Elder son to control (had his own agenda?) so he decided to manipulate with someone weaker than him.
I know these are not facts, it's just a hypothesis and something to think about. Feel free to correct me and suggest better options of who is Tobi.