yin yang creation out of senju and uchiha blood cells. from Madara Uchiha. he is a bijuu for he is made the same way they are. Madara created him to carry out plans to revive himself later. Tobi is not Madara just a form of him.
One thing is sure, Tobi isn't someone we don't know. if he turns out to be someone not already known to us, then the manga will lose all its excitement.
My guess would be :
Madara's Brother : Izuna
No i finally figured out who he is..........are u ready for this.......Its chuck norris behind that dam mask! who else could mess stuff up like that....His beard alone must have thousands of jutsu's
I have been thinkin n i guess he is madara's younger bro cos we know madara died after gainin the rinnegan(hence the edo tensei summon) so he cnt be madara n tobi claims that madara's younger bro died after his eyes were taken from him(to sauske) just like he claimed the kyuubi attack on the village sixteen years back was an accident yet we know it wasnt.
He cdnt be obito cos he is too old and he couldnt be shisui cos itachi claimed shisui died n he loved peace so i have to go with madara's younger bro.