Well, I don't know if this is my guess or rather just my hope... but I think we might see a Pike&Shot game, ideally covering the Rennaissance and Baroque periods (1450 to 1715 would be epic, right before the Fall of Constantinople and the end of the Hundred Years War and with the muslims still present in Spain, to end with the Treaty of Utretch and the death of Louis XIV).
I seem to recall that CA said they would not be making a 3d sequel anytime soon, so that pretty much rules out Rome, Shogun and Medieval. Therefore it would be either Empire 2 (which I find unlikely) or a new game set in a yet unexplored period. So there is a good chance that we could be seeing a Pike&Shot game in the near future. I don't think they can pull off the complex military tactics of the 16th. and 17th. centuries warfare with the current state of the engine, but given that they are going to have plenty of time to develop it all while they are still busy with Warhammer (taking into account that the second and third installments of the trilogy are not going to draw nearly as many resources as the first one), I could see the next historical game being a completelly new thing with a "new" engine. So why not. And what better "new thing" than those two awesome centuries, after all, the other options are too similar to what we have already seen (Victoria would be pretty much like Napoleon/Empire, an Charlemagne is essentially a late Attila, and apparently already in the making as an expansion, so...).
That is, unless they delve into 20th. century warfare, which I personally would not like at all for TW. It just does not fit WW2, and while WWI could work (they could be using Warhammer as training ground to test flying units, and they might use the centenary as a marketing hook), I'd find outrageous to see a WWI game before Pike&Shot, to be honest.