That's pretty cool.
Cell-shaded fo' sho, cuz anything else just looks like gay computer-based stuff. Cells make it more... artistic. The goal... should be to get beastly. And incredibly strong.
You should have it, like, the Gotei 13 is made up of players. And you can advance through the ranks, or kill a captain, or make your way to the top whatever way.
And zanpakuto creation should be... I dunno. I guess when you get to the level where you unlock communication with it, you get to do a creation for it. And like, you can use exp to unlock higher releases, or add custom effects to the releases you already have.
Why So Green And Lonely? We Are Accidents Waiting To Happen.