I don't get it why everyone hopes it to be Grimmjow... it would kinda spoil the whole damn Arc and show that Kubo wasn't able to make up for the failure at Final Getsuga and the Fullbringer Arc, since the Arc then would only again be based around Ichigo vs Grimmjow.. which we already had! This would be kinda boring, I don't know how others see it but I would like it to see an Arc that we didn't have before and not repeating damn fights.
I actually like the Idea that it either is Uryuu's Father, somone new or just none of the hell damn fights we have seen. There are so many people I want to see fightning a serious 1 on 1 like Isshin, Yachiru, Uryuu's Father (sorry.. don't remember the name) or someone who we haven't seen yet.. but most people hope for something old that already happened...
~sorry, but I am a total Anti-Nostalogic o-o