I think Madara 'cause he's evil like Indra, crawls for power, and wants to rule. Sasuke is not that guy. Even though he was like a villian, he turned out to be good and he teammed up with shinobi allience to defeat Madara. Now why would Indra do that? He would probably be on Madara's side and then somehow he would kill him or something. Plus, Indra's hair, clothes and attitude are same as Madara's, just like his Susanoo too, so no it's not Sasuke, it's probably Madara, cause kishi wants us to think it's sasuke just to make a huge plot twist and make madara narutos true rival