I don't think it's too far fetched for Tobi to still be Obito, though I agree that it would be pretty lame for him to be the ultimate villain without a damn good reason.

Obito was killed on a mission in the grass country, and I think it'd be pretty safe to assume that Zetsu is from the grass country. It's possible that Zetsu rescued Tobi from underneath the rock which would draw a connection between Obito, Akatsuki, and Zetsu. Don't forget that Zetsu was the one who "brought" Tobi in to replace Sasori, furthering a possible connection between them. Who knows why he would go to this sorta length to hide himself as the true leader.