Re: What could Aizen's Bankai could've been
His Shikai's abilities from what i've come to a conclusion of is:
Kyoka Suigetsu in Shikai mode can, create an illusion of something.
We have only seen it copy other beings.
What he creates, is almost an exact copy of the being he decides to copy
for example himself. BUT it has some weaknesses.
1. By touching the blade, you can end the illusion for yourself.
2. You can see trough it!
How can you see trough it?
When we went back to when Shinji and the others where captains, Aizen told Shinji
that the only reason why Shinji was deceived by his illusion, was solely because he took distance from Aizen himself.
If he had known Aizen alot better than he did, he would have noticed the small differences, as Aizen's Illusion can't
recreate EVERYTHING INSIDE the said person, though it can create some of it.
Now that we have that cleared, i'll try and explain what i think his bankai is, if he has one. i'm basing my guess with his shikai in mind, out from my description.
I think his Bankai is a area of effect bankai, so to say.
for example in the first arc, we see Kenpachi Zaraki battling it out with Tousen and Komamura.
When Tousen tells Komamura to step back, he releases his Bankai, and it's a whole area that gets "sealed".
Inside this area everything is black, and all your senses are taken from you.
(not sure if it's black, or just looked like it, cause the vision senses was taken away as well)
Now i think Aizen's Bankai is similar to this. in this area he may be able to create ANYTHING literally.
it's his own personal universe where he can do whatever he wants basically.
Though it's all illusions, i don't think it's the same way as his shikai. i think these illusions are something which doesn't necessary
have to be real, for example, he could create heaven and really have the affected people inside the area think they're heaven, though they're not.
meanwhile they're inside this area the affected users are not able to do anything, unless they can break trough the illusion, or someone else saves them.
i haven't thought much of the exact details, though i could if someone wanted to know more
Another idea i have, is that he doesn't have any bankai, that he just stopped when he realized how strong his shikai was. he simply became too arrogant. in some way i highly think he is in the same state that Kōga and Muramasa was.