Well it comes down to a sequel or a new period altogether. Here are my thoughts on both:
Sequels: Honestly I would absolutely love to see a Rome II. Medieval was a favourite of mine, but that's had two games so he can back off for a bit. I've never understood the hate Empire received as I enjoyed the change of pace from swords to guns, but perhaps that's just me. Nevertheless, Napoleon was a pseudo-Empire II, so we don't need to revisit that, yet, either.
New Period: This is going to be more of a wishlist than a specific pick.
Varied and large Geography:
Shogun 2 seemed small after the sprawling lands of Empire (3 theatres) and Medieval II (seeing as America opens up in endgame). It is the fact as well that, unlike Europe, Japan is just this kind of straight line from one end of the map to the other. Sure you can get in a boat and sail and there is a little island on the west side but it is still dull.
Something that involves a blend of cultures works best:
Again with Shogun II here. I did enjoy the game, but variety ended up cutting my total play hours short. Every team was more or less the same, with some minor differences in stats (Shimazu special unit being a badass version of Katana Samurai. And what was it? ...drum roll....SHIMAZU Katana Samurai...yawn...) It might be an easy game to balance the factions, but made it as boring as hell to play more than once.
Does not need to be familiar, only bloodthirsty:
If Shogun has taught us anything it's that we don't need to be spoonfed something we know about in order to find it interesting. (For example the constant Americanising of world folklore for American audiences in Hollywood). I personally think that's kind of an insult to the American viewer, but then I don't commission expensive studies to analyse consumer behaviour. Just give us the chance to field huge armies that butcher each other over 20-40 minute periods and we're happy.
Something unique:
In a way, this counterpoints the previous point. Sure we want bloodlust (That's why we buy a game called Total War) But we also need something that makes it special. Something that makes it so it's not a basic reskin of a previous game with different maps. As long as the period/place allows unique mechanics, then I am sold. (Think Shogun's Ninja assassinations; Medieval's Royal marriages and Papal Decrees; Rome's three families vying for public adoration and political success; and finally Empire's revolution dynamic).
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Anyway. Those are my four main points I wanted to make. As long as it doesn't go further forward in time than Empire (as the RTS element would not really work in that setting) then I can almost guarantee a purchase from me - CA have built a lot of positive karma with me and I am willing to try almost anything with the Total War name behind it, trusting they will do a good job. Something Sid Meier also did before that SHITE that Was Civilization V. But...that's another story...