As with a few other posters: Rome 2, where I think the replenish mechanic from NTW & TWS2 makes more sense, due to the mercenary nature of warfare during that period.
Other than that, I'd like to see one of two things:
1. Hellenic (unfortunately, this would be quite boring for some people, what with just hoplites and phalanxes, but I think you can get enough variation among the factions, but with ETW, NTW & TWS2 having almost identical rosters except a few different stats and unique units, it's not going to be that different).
2. Ancient China (and I don't care what dynasty, purely for the aesthetic and battlecries in Mandarin).
One thing I will say is that it's almost impossible to get a good game out of anything post C19th because of the much more fluid dynamic of warfare. TBS/RTT is all well and good up to and including the German Wars of Unification, but even then, the army movements and engagements become far too unwieldy for a game like TW to deal with, since operational strata come into play. WWI is difficult enough as it is because the RTT aspect will play like arse. WWII will be borderline unworkable because army movements cannot be portrayed accurately, and it'll take a mammoth graphix engine to be able to pull off proper tank battles in a full set-piece battle (and by then, battles weren't really set-piece at all) and on battlefields that are of a size that can be effectively represented. Pure aerial battle along the lines of Battle of Britain will be a tough ask as well, even more so if it's in a combined arms sense. Only naval battles will be any good, with a dual interface for aerial/ship-to-ship combat, but from a strategic perspective, the faction that builds all the aircraft carriers will win by default (USN/IJN/RN in all likelihood).
I'll shut up now...