I have some theories, apart from the obvious.
Yeah, the Magician guy, Grimmjow, Ishida's dad, and the former Kidou corps captain, they all sound pretty plausible... but... I think it might be that Shinigami guy Rukia met when she first entered Hueco Mundo. He followed the party all this time and was able to stay hidden because he's had all those years of practice in hiding his spiritual pressure... And he followed Aizen somehow...
Anyways... with how powerful Aizen has become, I think there really is only one flaw with his plans - his arrogance and reliance on himself. Aizen thinks he is the true master of the Hogyoku and that he is using Ichigo and perfecting Ichigo's powers in order to eat him and increase his own powers later. The truth, however, is somewhat different.
Aizen said it himself - the Hogyoku is intelligent, and not even Aizen realized this until he became its host, but what are the implications of this? What if it is not Ichigo, but, in fact, AIZEN that is being used? I think the Hogyoku really wants ICHIGO to be its true host and has been using Aizen to help evolve Inchi for this purpose all along - and it is making Aizen think that he is in control.
Aizen is the perfect tool - because he is so arrogant, he does not even realize he is being used - and won't until the last second of the final battle, when the Hogyoku betrays him, robs him of all power, and offer Ichigo all its powers instead... Then Ichigo will kill Aizen, but will reject the Hogyoku's powers and will have Orihime destroy it instead.
The end...