It seems everything will be down to Kenpachi, Yamamoto, the Vizard Captains and possibly Unohana. Well, and Ichigo whenever he manages to escape/gets released.
I have the sneaking suspicion that Aizen is somehow going to get involved - whether it's Soul Society releasing him to fight out of desperation, or him somehow breaking out during the chaos.
...or maybe the whole invasion is being orchestrated by Aizen! You can never count that sly *** out.
I can't wait to see Unohana fight - she's supposedly the strongest captain after Yamamoto. I wouldn't count Harribel out either. And Grimmjow. Come to think of it, I wouldn't be surprised if the person who pointed his sword at Urahara is Grimmjow (i know that Arrancar aren't supposed to go back to their normal forms after they've released but gogo retcon!).