If you want fully-fleshed out characters but don't want to make them [i]all[/i] beforehand, maybe you could just create the characters as you come across them in the story. Flesh them out only as they come. That way you won't have myriad unused characters just sitting there. That's generally how I make my NPCs, which is probably not the best but works for me. :P

Anyway, it sounds like the scale you're looking for would fit much better into the [url=http://www.roleplayerguild.com/forums/42]Nation Roleplay[/url] forum, in case you didn't already know that. I'd recommend looking at how some of the longer-lived ones in that forum do their structure and army management, because it sounds more or less like what you want, except perhaps on a more macro scale. You could potentially combine the normal roleplay strategy with the macro scale strategy with the leaders doing battle with big baddies and occasional comments or even focuses on the army's (or whatever a unit of fighters would be called, it doesn't really matter) progress/status in battle. It would probably be hard to keep track of, but if you're determined and organized enough with your notes, I think you could do it.

But yeah, interest is always the hardest thing to garner. Maybe if you turn it into a more general RP and use all of the elements from Bleach (explaining them along the way), you could appeal to a wider audience than just Bleach fans?