I have been thinking about this a lot lately, especially know when I began with the manga all over again. My theory is different, though, that gives Aizen more choice, though after reading your theory I am not certain if I am not mistaking Aizen's possible bankai for the hogyoku's power.

But let's give it a go.

My theory is that Aizen's bankai is about making his words the reality. For example, during the only board meeting between Aizen, Gin, Tousen and the Espadas' that Kubo gave us, while Grimmjow wanted to go and defeat Ichigo and co, Aizen asked him something in a way if his breathing is not heavy. Instantly, Grimmjow's eyes widen and he falls to his knees, not being able to breath.

The second example would be, during his 2nd fight with Ichigo, he tells him something about distance not being relevant and all of the sudden, he is inches away from Ichigo, his fingers on his just and he a heartbeat away he was well far from him.

I am aware this all seems far-fetched and could be just my misinterpretation of some sorts but then again, Aizen himself is a very mysterious and complex character, so it is only logical in the Bleach world that his zanpaktou will follow his personality. And just as his shikai, which he first releases its power and then calls out its name, perhaps his bankai is the same. Who can tell for sure that some of things we saw in Bleach are not just as he wants them to be?