Battle condition: Aizen has all his powers shown in the manga/anime and is at his best in all fields, that means as smart as he was as a captain and as strong as he was in TYBW despite the picture.

Plot: In this timeline.... Aizen defeated ichigo in their battle in the real world, and proceeded to seek for more knowledge on how to get the key to reachthe soul palace which landed him at Namek... He went to gather the dragon ballz in namek to grant him said wish...

Then suddenly appears another entity seeking the dragon balls (Momoshiki) in order to gain power to rule over all other otsutsuki....

Momo takes a peek at Aizens fate and realises how much of a threat Aizen is and will be, then realised he has to beat him.

VS

So which three eyed, long haired villain with white attire is taking this??