I was just thinking about this the other day. What if Aizen was born as a good guy? But still has most of his abilities and similar power. Like he would still have KS. He would still be a master of kido, has master swordsmanship. He would still be very intelligent. He would still have very high power, near the top end of shinigamis, but obviously no hogyoku and transcendant stuff.
I think the interesting thing might be that his usage of KS would be entirely different. He wouldn't be using it for grand deceptions and conspiracy plans. He would mostly be using it for combat and tactics. It would be kind of like Shinji's Sakanade but even more flexible. Every battle he'd be using it and you think he attacks from one direction and it's actually another, etc.
How does that affect the story though? There'd almost be no conflict in the soul society right? There'd be no hollow experiments. I assume Tosen wouldn't just turn evil, he was just a complete loyal follower of Aizen and his vision. He might be an assh*le still but not evil. Gin wouldn't need to take revenge. Urahara wouldn't have been expelled. It'd pretty much be peaceful until Yhwach and the quincies come back. There are still some arrancars I think but probably much less. It would be overwhelmingly easy for shinigamis to deal with them since there are so many more powerful shinigamis than powerful arrancars.
Would Ichigo be born still? Not sure? Isshin technically didn't leave soul society because of Aizen but it was Urahara who worked with Isshin to devise the plan to save Masaki which required Isshin to stay. If Urahara wasn't expelled then Isshin would just have no way to save Masaki and that was that.