I knew Isshin wasn't part of the plan. Of course Aizen changes it up. You'd have to say Masaki will be another surprise. How the hell is she going to drive them off? Unless she just shows up extremely late? I am finding my self liking her though, she's a good character. Smart and silly.
I wonder if Urahara will show up too? It's all going on in his hood after all and we know he always knows when shit's going down. Not to mention the Visored could also show up, cause when we think back to earlier in the series Isshin did already know who they were, so perhaps they had met here?
Edit: I'm also inclined to believe that Isshin is stronger than that Hollow. I don't see why Aizen would risk going out there and being discovered if he felt the Hollow wasn't going to lose. Will we see the Engetsu Bankai?!