Back before the mask came off and it was revealed after all these years to be Tobi, this forum was crazy with speculation so...although we now know him to be Obito, who would you want him to be?
I, given the choice would've honestly made him a future Sasuke. They should've focused more on how Tobi made Sasuke his student and encouraged Sasuke and Naruto to fight- I always found tobis interest with Sasuke to be suspicious, it could've been a future Sasuke or a parallel world Sasuke (Kamui is mysterious and could've easily explained this) it also makes the prophecy make sense and would start to bring things to a close.
My favourite theory was that Tobi wasn't actually anybody. The masked man that people would see was actually just a clan of different people all with the same mask. This is why they all seem to have different personalities and different costumes and fighting styles. They could've all been UCHIHA survivors or a cult. Fugaku would've been the one with the chains that fought minato, the Good Boy Tobi was Obito, the long haired Tobi in the hidden mist was Shisui or Izuna and the war costume was Madara. How cool would that have been?! And it also explains every single plot hole and asspull
And the best thing was:
Everybody would've been right!