Yeah, that's the original point I was trying to bring up when I posted the current Embodiment list. Though, it lists Drake as an Embodiment (which last I checked, isn't he an Extra stage boss? Maybe he can be Slaves' partner?)
Also, Stage 2, 3, and 4 all have potential spots for midbosses to add more characters. Stage 4's midboss is currently listed as Tenshi for some reason, but in the interest of fitting in more characters that should be in we might want to switch that out for someone else.