Amodelsino said:
Not to mention he's nowhere near the level of a Captain, and Yamamoto himself said the primary role of the Captains was to be Soul Society's Defense Force. It's why any Shinigami who can kill a Captain in a legitimate fight is given their job.
That, and you kind of have to be a Shinigami to be a Shinigami Captain.
The Rule that says "kill them with 200+ witnesses to take their job" technically don't state that you have to be a Shinigami to be a Captain.
At least, not in the translations I've read. The rest of them say that, or require that you at least have Shinigami abilities.
I remember a fanfic that took that idea and ran with it. Basically, a
very bored and powerful Vasto Lorde invades Soul Society (implied to be Ichigo if he became a Hollow or something, but not explicitly stated), goes to a very public area, and then kills Aizen for shits and giggles in front of everybody.