Here's all the people from the poll in this thread and why they aren't Tobi. Keep searching!
1) Juubi: No. Juubi already has a body and is being revived as we speak. Plus, it's somebody that Madara knew BEFORE the Juubi was revived.
2) Sage of Six Paths: No. Completely different motivations from Tobi. The sage wanted peace through love and mutual understanding, not forced peace through conquest by genjutsu
3) Izuna Uchiha: So Madara eventually got old and died, but Izuna didn't? And how does Izuna seem to have sharingan-based space/time techniques but Madara doesn't, even though Madara has Izuna's eyes?
4) Kagami Uchiha: He's the right age, but that doesn't explain the space/time techniques or the weird zetsu-like regeneration thing he's done.
5) Obito Uchiha: No. Wrong age. There's no way he became so powerful in the brief time between when he died and when Tobi fought Minato. Also, Obito had one eye left, but Tobi clearly had two in the fight against Konan.
6) Shisui Uchiha: No. Completely opposite ideology and motivations. Shisui would never start a world war.
7) Setsuna Uchiha: Setsuna was part of the plot for the Uchiha revolt. He wanted the Uchiha to be free from Senju rule, but then Tobi massacred the Uchiha. That's sort of exactly the opposite, right?
8) Madara's Son: Did Madara have a son? On what evidence? And where did he get those space/time techniques if Madara doesn't have them? And why doesn't he seem to have an MS or EMS (or even a self-unlocked rinnegan) if Madara knew all about unlocking those things?
9) Obito's Father: Tobi has never once mentioned Obito. He doesn't seem to have any kind of familiarity relationship with Kakashi. Doesn't act sentimental when seeing Obito's eye in Kakashi. Hasn't tried to use Edo Tensei (or Rinne Tensei) to revive his son, both of which would be possible (especially if he killed kakashi and stole that eye).
10) Madara's Split Personality: Split how, like a clone? Tobi and Madara seem to have different personalities. Madara is more arrogant, insulting and boastful. Tobi is more quiet and self-confident. They don't really look the same, don't act the same, and Tobi doesn't have all the techniques that Madara has. Plus, Tobi has techniques (zetsu-like rejuvination, space/time techniques) that Madara does not have.
11) A copycat whose role model is Madara: Sure. But which copycat? The only thing he's copying is the plan, because none of his techniques are the same.
12) Real Madara in Zetsu Body: No? Madara's spirit/mind are in the Edo Body. So if those things aren't in Tobi, and if the body isn't Madara's, then how is this possibly Madara, again?
13) Some one from Madara's Time: No. They would be way too old. Madara appears to be older, like 50s or 60s. He doesn't appear to be as old as the tuschikage or older like he would need to be.
14) Elder son of Sage of Sixth Path: Then why does he have sharingan and not rinnegan like the elder son had in the flashback? Why did he need to steal a sharingan from Nagato? How did he live so long? No, it's not him.
15) Zetsu: Like, he is Zetsu, or he's a clone? Then why does Tobi give orders to the "real" Zetsu and not the other way around? Why create an army of clones of Zetsu instead of creating an army of clones of himself?
16) First Edo Tensei: Made by the second hokage? That does explain why he appears to re-grow limbs and why he appears immortal. It even explains why he seems to have an unlimited supply of chakra and why Itachi would call him a shell of his former self. HOWEVER, there are some things it doesn't explain like how he ripped out his old eye to replace it with a Rinnegan without it regrowing immediately, or where he got his unique space/time jutsu. Exactly who is it that was summoned with Edo Tensei to gain all these powers?
17) Tobirama: Where did he get the sharingan? How has he lived so long without growing very old? Why doesn't he seem to have any of the facial features or the white hair of Tobirama? How was Orochimaru able to summon Tobirama for Edo Tensei, if he wasn't dead?
18) Another Senju: Where did he get the sharingan? Where did he learn the space/time jutsu that no other Senju has?
19) An Uzumaki: Why is his hair black, not red? Where did he get the sharingan?
20) Tobi: Yes. Tobi is Tobi.
21) The ramen guy: No.
22) Someone other than these: Obviously
23) I am done. No Theory: Great, I'm glad to hear it.