Grimmjow isn’t that strong. He can’t slice Kirge in half if Allon barely nicked him and Ichigo only had so much damage on him. Neliel isn’t that strong either. There’s a reason Kisuke looked shocked as hell.
It could be Aizen. I just don’t like the idea, because personally I thought he was a boring villain. “I planned everything.” And the fact that he’s immortal means it will probably take some kind of deus ex machina to defeat him. The Deicide Arc was not one of my favorites.
I’d rather see Gin come back. That would be more interesting.