Ok, just something that seems likely to me given the current circumstances:
Aizen himself - insanely strong. Really cheap initial release, enough spirit force to reduce an espada to their knees, taking out captains in one shot while barely using any of his abilities, the fact he is *probably* a vaizard by now (he's had access to the hogyoku for ages and said it was his goal to master hollow powers) etc. etc. I only see a couple of scenarios for his demise:
1. Ichigo (or whoever else) gets just about the biggest powerup imaginable, and I don't really see how this could happen right now. His hollow powers are only going to get him so far.
Or alternatively, he fights Ichigo, but holds back loads and makes very little use of his massive range of moves.
2. Orihime kills him - her abilities aren't based on strength, they're based on her own intent, and judging from how he kidnapped her and didn't think her a major threat to him, it seems likely she'd be in a good position to take him by surprise and kill him. Especially as he's spent quite a bit of his time bashing her friends about, and not to mention the fact her offensive ability has seen very little use so far (besides getting smashed), which would make it all the more dramatic when it actually works.
3. Ichigo talks about his resolve for a bit, flashes some blue stuff around, big explosion, Aizen dead.
Personally I think it would suck if she did kill him, as it would most likely happen without him using his full abilities (which I'd really like to know)
But that could be another point in favour of the Orihime theory - perhaps he doesn't have a bankai (that we actually see) because she's going to kill him before he actually uses it. Which would make some sense, I'm at a loss to think of what would be more dangerous than that complete hypnosis thing.
Anyway tell me what you think, I'll add more as I think of it...