I think it is Madara also. Seriously, throughout time it is said that both of them have been reincarnated or whatever you want to call it into other people, and they have waged war. Well Naruto isn't going out to fight Sasuke, he is going out to fight Madara, who if obviously a lot stronger than Sasuke. Somehow Indra will show himself through Madara when Naruto goes out to fight him, and Indra will recognize Ashura inside Naruto and then Naruto can somehow change him, or make it so his chakra won't be reborn into another person. Either that or I guess it could be possible that Indra is in Sasuke, and then will finally realize love is the way to go and help his "brother" defeat Madara. I think the other way makes more sense though.