I always wondered if the Quincy arc was originally supposed to come after the Sould Society Arc. Remember, while Hueco Mundo and Arrancar were planned to some degree (we see them early), the entire conspiracy plot is the SS arc was only there to make it less monotonous, so Aiten got almost randomly introduced and Kubo only decided in the last minute that Aizen's murderer was Aizen.
So maybe originally, after rescuing Rukia there might have been some breather arc (maybe the Hell arc) to make sure that the Shinigami and Ichigo become friends, followed by the Vandenreich invasion. This alternate Vandenreich might hide in Hueco Mundo (the last place one would expect Yhwach in) and use Arrancar as foot soldiers, explaining why Kubo felt the need to bring up that the Quincy enslave Arrancar in TYBW, but then ignoring it altogether.
There might have been MUCH less Quincy then, maybe only the Sternritter. Aizen's and Yhwach's initial goals were rather similar, but because Kubo still wanted to do the Quincy arc Aizen wasn't allowed to be successfull. None of the uber-strong captains were allowed to show off for the same reason.
Finally, I also think originally Kenpachi was supposed to be Yachiru's sword, it would have made SO much more sense. Just so much.
But because Kenpachi was too popular he wasn''t allowed to.