Hmm.

Q: Would people like to be able to create unique histories and their own lore (royal families, factions, races) at some point in the RP?
A: Well, sticking in nobility seems fine. Maybe giving some of the squads (plausible) specialities. But making up whole factions and races? At that point you're not doing a Bleach RP, are you? xD

Q: Would people like to have the chance to lead their own Arcs later in the RP?
A: Side-arcs, sure. Noooot huge plots that lead to the GM not... GMing.

Q: As Bleach is a verse centered around conflict between factions and inviduals, it stands to reason that an RP based on it will also be as such. The question is, what races do people prefer to be playable? Shinigami, Arrancar, Quincy, Spiritually Aware Humans, etc etc.
A: Shinigami. You have a nice organisation where people have a reason to know each other, and they're in a central position to deal with absolutely everything. Good power variety without being too 'and now what?', as well.

Q: Would people prefer the story's villain faction to be playable, or ought they be controlled by the GMs entirely--or is a middle ground best?
A: Mostly GM's. It just... drags if players have villains as well, as then there's likely to be more stagnation and undefeated villains (especially when, with Arrancar etc., you [i]expect[/i] them to die off)

Q: Should there be a limit to how many characters a player can make?
A: No hogging top spots? Obviously there's a reasonable limit to how many flunkies you can consider actual characters, though.