I'm mostly not sure how much needs to be set up - basic rules, naturally, and some settings/items/whatever*, but I don't know where the balance between too much and not enough is.



* So far, the basic idea is that this is the Omega Crew of the Seireitei - the place where all the divisions dump their misfits and losers**. They got sent to the real world on some sort of mission and basically got forgotten in the mess when Ichigo et al. invaded. So they're stuck in their secret HQ, being run out of a convenience store (see the dire Bount arc), and subject to whatever malfunctioning equipment/gigais/whatever they've got, with the occasional Soul Society person throwing a wrench in the works (i.e. the only canon characters would be mods, because I like the idea of Yachiru messing with their heads.)

** Even more so than your normal run of shinigami, yes.