I'm hoping for 1 or 2 expansions for Shogun2 before they move on to the next title.
The candidates for expansions as I can see are:

1. Korean invasion. Really interesting campaign, with both Koreans, Chinese(ming) and Jurchens(manchu) thrown into the mix. This game would look more into handling limited resources of men and supplies, and securing and maintaining supply lines for resupplies and reinforcements, rather than developing cities and recruiting troops.

2. Mongol invasion. Return to the original expansion-topic, so less likely, however they could do it better, and more interesting this time.

3. Nanbokuchô era. Age of the southern and northern courts, a more consolidated all-japanese conflict with less contestants, set in an earlier period. This campaign would allow CA to focus more on story, intruiges and roleplay than just pure strategy, if that's something they want to look into... I suppose the Genpei war is also valid for such a setting but meh, too hyped and nagged out IMO.

4. Boshin war. The destruction (or not) of the shogunate and the restoration of imperial rule, phasing out of the old ways and the samurai caste and introduction of modern military/weapons. It could also feature the struggle of the real last samurai; Saigo Takamori, and his inevitable downfall. Really intruiging setting, however it would require quite some focus on 19th century firearms/tactics to be done right.

After the Shogun2 expansions, I'm hoping for any of these:

1. 17th century europe, the age of pike and shot, hopefully focused on the 30 year war. More focus on military campaigns, fighting battles, logistics and diplomacy than country development, research and recruitment.

2. Mediterranian, 8-5th century BC. The age of greek colonization. This game would simulate the conflicts in greece leading to colonization, and the establishment of colonies and their struggles. It would also feature conflicts with other local powers, the encroaching Persians, the rise of nations such as Rome and Carthage, and the conflicts between greek civilization and these emerging threats. IMO this period preceeding the Persian wars, Peloponnesian wars, and the rise of Rome is just as interesting, if not even more interesting than the latter. But clearly, this game would have as natural expansions, all these popular conflicts.

Or,

3. Mongol invasion PROPER. Starting with the rise of Genghis Khan, his consolidation of the mongolian tribes, continuing with his and his successors conquest of one part of the world after another. The challange for players would first be to emulate his success, and even outdo the Mongols by conquering regions such as India, Africa, Japan and Europe. This game would really offer a grand scale to TW, and present scores of diverse factions, either letting you conquer them as the Mongols, or resist and compete with some of the smaller factions.