WFB always struck me as the dorky little brother of 40k. It's like a step above generic D&D, but a step below the wild absurdity of 40k (also, for me, sci-fi > fantasy). I'm enjoying WFB in the context of TW, though.
I sure would love a 40k TW with some Tyranid action, maybe provinces become planets with several major settlements, etc. They've already got flying and tank-like units covered, shouldn't be too challenging to do with the TW formula. Sure, they'd no doubt release Space Marine chapters as paid reskins/palette swaps, but fuck it.
Something else Total Warhammer is missing are actual trade routes, where you can see merchants walking back and forth, but to be fair that's not exactly a necessity for a Warhammer game. I'm personally glad that the naval combat is gone because of the insufferable tendency of the AI to spam a hundred little canoes and other nonsense that made it not-fun.