Well I'm going to go ahead and put my two cents in this because I've been looking for a bleach rp to play again. My character never got a chance to blossom and I owe it to my character.

As for the questions themselves:

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=royal families, and factions yes races that's iffy because of the depth you would have to go through to explain how they are part of the story. For example if your playing as a arrancar and the current arc is about the Shinigami and their political issues then it makes it impossible for that person to really play into the story.

Q: Would people like to have the chance to lead their own Arcs later in the RP?

A= of course! I don't think their are many bleach fans who don't watch or read blah that don't want their own story arc

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.

I think it all depends on the story the gm wants to tell. I prefer visor but that's just me 

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=at the gm discretion best they control the villain unless this villain is for a single characters story arc in which case I suggest a middle ground of how it will play out but the gm will control the villain.

Q: Should there be a limit to how many characters a player can make?

either one per faction or a limit of 2-3 characters. So that way if someone can't post the entire story isn't at a standstill