I don't think he will come back someday considering how old he is. It seems this is his last moment in his beloved club, Real Madrid.
Best of luck Ancelotti. You have done great jobs, though ended in the bad way.
This is actually a big one for Carlo ancelotti because i am quite sure that this is his last contract in his coaching career, and considering his age, it's logical to say that national team football is what's best for him now because he would have a lot of time for himself and his family, unlike club football that comes thick and fast, so it's a welcome development and a big one for him.
Actually, it's a great deal for Ancelotti, Brazil isn't a simple team at the world level, and his ability to perform better with the Brazilian national team would gain him worldwide exposure. He had it all thought through before considering this move, so, it's best not called a bad deal, and aswell he's aged enough to retire after the world cup or maybe settle for a smaller division where he'd continue with his coaching career.