My tobi theory is that tobi is zetsu.
I recently just had this epiphany moment in which i realized tobi is zetsu.
What i think happened is that after Hashirama and Madara fought, zetsu absorbed(by eating)the corpses of Hashirama and Madara which would explain why tobi has both senju and uchia dna. Also because the white zetsu part can be reproduced by the 1st dna meaning they have similar characteristics. After all why not in third databook it says "Zetsu's hobby is observing unique shinobi." Which shinobi are more unique than them
Zetsu is seen having split personalities between his white and black part. Tobi also exhibited this with his introduced personality that was
childish and hyper. While later he changed to a serious one like that of black zetsu.
Tobi is seen with really similar healing powers to zetsu. It appears that
he can lob off body limbs whenever and just grow them back, each time he did that the limbs looked liked they oozed out white liquid similar to zetsu.
Closeness to zetsu
When tobi was first introduced tobi clinged to zetsu. They seemed to know
each other for a while and zetsu actually got annoyed by him like with white zetsu. In the Third databook it says "Wherever 'Tobi' is, Zetsu is never far behind."
Zetsu appeared at the statues before to observe naruto and sasuke fight.
Zetsu's age was never revealed.
Zetsu appeared with Hashirama's skin color in the anime.
Hair style when maintained is similar to zetsu.
Zetsu said he would share the itachi Vs sasuke fight with tobi even though he was not there as if they were connected somehow.
A former problem with this theory is kakashi style techniques, but he said that he got the sharingan during the third shinobi war in which he could have got from obito.
Only remaining hole
Revived madara exhibited fire techniques that tobi never used yet according to the manga.Only earth style but that does not mean he does not have it.
obviously tobi is not the exact same Zetsu. Maybe a clone or another of similar techniques. Also zetsu would not have a motive but maybe madara possessed his consciousnesses by use of the sharigan the zetsu obtained.
despite believing this theory I in no way believe zetsu is the master mind of the akatsuki.
i actually feel somewhat disappointed that tobi probably isn't obito which was my first theory or izuna my second but zetsu makes more sense.