Thing 1: Aizen is most likely DEAD!! From what we last saw with Juah and Aizen, he refused Juahs offer 2 join him and was off-paneled..
Thing 2: Grimmjow will probably NEVER return... these Quincys FAR outclass the hollows. what could he possibly do when he wasn't even the strongest arrancar?
So you can all STFU complaining about the lack of appearance 4 those 2 characters....they had their time 2 shine and if u miss them so much, go and re-read the abundance of chapters in which they appear. thanks for reading