What you suggesting is that we will not see his bankai, but not that he don't has it. I agree that we will not see his bankai, because his shikai was already completely hypnosis, and you can't be better than completely. That's where Kubo has written himselif into corner.
It's also possible that Aizen used his shikai to fool others into thinking he has bankai. But I found that not very likely.
It wouldn't make much senses for someone like Aizen, who is one of the most talented shinigamis in manga, to not posses bankai.
Btw, Aizen already said that Ryujin Jakka is strongest zanpakuto and by what we say I would also say that Ichibe's zanpakuto is stronger.