Starting to run out? They've already done each of the periods twice. So yeah, it's really about time they got some fresh material. I'm surprised they haven't done anything with the Early Modern yet.
As for the collab with GW, it probably won't be a straight up Total War game, but rather something that adapts the WHFB rules to a Total War -style game. Certainly an idea with a lot of potential, since with the units of soldiers and everything it's already pretty close. I know there was an old Warhammer RTS game like that back in the day (before the first Shogun, even), and some newer one as well, but I never got around to playing them.