...Nawaki to steal the Uchiha limelight?
How many people expect Tobi to be Senju? Probably none. Fools all.
To start- are any known characters still eligible to be Tobi? I'll try to list some:
-...I can't come up with any more.
I personally haven't seen any theories about Tobi being Nawaki, so I'll put forth the idea just in case it hasn't been brought up.
I don't know what his motive could possibly be to betray the leaf village and forsake his dream to become hokage, but Nawaki was conspicuously absent from the edo tensei ranks. Sure, Nawaki was weak and young, but maybe his 'death' was a clever ruse used to mask his desertion- the real reasoning for him not being summoned being that he isn't actually dead.
Nawaki could trace his lineage to the first hokage- living Senju would realize the power of the name 'Madara Uchiha'. Nawaki also had many years to secretly acquire sharingan, and then 'rule' the mist village by proxy. His death was even 'off-screen.' The down-time between Nawaki's disappearance and Tobi's assault on the leaf village with the 9 tails creates a window of oppurtunity for Nawaki to explore the world, learn about the infinite tsukuyomi/10 tails, and even start the akatsuki.
Also, my biggest point (coincidentally my only real reason for this theory) is the way Tsunade mistook Naruto for Nawaki. Naruto and Nawaki were said to bear an uncanny resemblance. Wouldn't it be shocking for Tobi to remove his mask only to show an older, wearier version of Naruto's own face? At first everyone would be shocked, then realization and confusion... it seems as likely as anyone else.
It would also be an interesting twist to see a Senju like Nawaki further try to discredit/disgrace the name of his ancestral enemies, the Uchiha clan, by stating that he's an Uchiha himself and starting a war in their name. Then we'd see that the Senju really aren't the wise, pragmatic leaders we've become familiar with.