Kira, why would Aizen want to die in the first place? I would hardly be able to believe that all he wanted was to die in the hands of some1 whose power he couldn't even imagine, it's more like he wanted that power for himself and yet was missing something important from his equation. Ichigo plays a role in that equation to him gaining such power.
It's more like Aizen is gonna pull another Aizen on us, saying that it was all planned, he planned to be sealed up since he knew HGQ wouldn't let him die, and he wanted to see how far Ichigo could take things the natural way and learn from that.
As for Aizen preparing Ichigo for a far greater threat, then the whole soul society is f**ked cause they cannot compete, then they're all living in an ignorant bliss fighting small time hollows when only Aizen was able to discover a threat greater than himself to all the realms.
Only thing I can think of would be if Aizen gave the Vasto Lordes (who would be captain level anyways), powers of the arrancar, and they grew beyond the levels of a Vasto Lorde arrancar even, and he's trying to stop them now.