Well, tbh it could be anyone just posing as Madara.
They don't even need to be from the uchiha clan..
It would be cool to see how Kakashi reacts when he finds out that it is obito (not saying that he is)
Also, Kakashi has a some-what similar ability to "tobi"
Kakashi can use space time like when he used it against deidara.
if it was somehow obito then he would have space-time abilities as well and maybe he has had more time to train his ability so that he can transport himself.
It DOES sound pretty ridiculous saying that it is obito, but nobody knows that it isn't yet and I'm going to stick with my thoughts.
I'm mainly backing up this theory because i would like to see how Kakashi reacts when he finds out the obito, He's best friend from back in the day who he thought had died is really the one behind it all.
So, until i know 100% that is cannot be obito no matter what.
Nothing will change my mind. Sorry
Also, for the people that say that he couldn't have survived half his body being crushed.
Perhaps there was someone who had helped him and and healed his wounds just as lady tsunade did for lee.
Also, "The right side of Tobi's face looks as if it is wrinkled"
What if it wasn't wrinkles but in fact the damage he had taken from the boulder landing on his face.
No sure xD, It's very puzzling..