Would just like to say i've been wanting to get back into Bleach for awhile now, so I'll give my input.
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?
Families and such like the Kuchiki bloodline etc. is fine I think, it adds depth and history that wasnt there otherwise but races are already in excess especially without a large playerbase.
Q: Would people like to have the chance to lead their own Arcs later in the RP?
I dont really have an opinion on this other than leave it up to the collective group to discuss i suppose.
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.
Again without the playerbase behind it you have to think small, Shini and arrancar. Then maybe introduce other races as the story progresses maybe arc wise. Even then if they wont be an integral part of the universe you create it may not be worth it to make that race playable.
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 mix of both can do well event wise, such as the introduction of a new villain, plot, theme -- the GM can step in to guide the rp along as fit. There has to be a certain structure since you have multiple players come in with their own ideas on their characters progression. which is fine but en masse can become unorganized.
Q: Should there be a limit to how many characters a player can make?
Maybe 2-3 initially and if the load isnt too much they can possibly add on. However I think additional characters after that should be limited in how often they appear or are played as.