Since 2000, British developers Creative Assembly have been cranking out games in their Total War series of strategy wargames. The Total War games feature a grand strategy layer, where you expand your empire, develop cities, assemble armies, research upgrades and new technology and then move those armies around into enemy territory. When battle results, the game switches to a 3D battle map where you can take direct control of your armies.
To date there have been eleven games in the series (Shogun, Medieval, Rome, Medieval II, Empire, Napoleon, Shogun II, Rome II, Attila, Warhammer and Warhammer II), nine major expansions and hordes of smaller DLC. The franchise has sold over 20 million copies and even spawned a BBC television series, Time Commanders, featuring players testing their wits against one another in battle whilst historical experts advise them (and are usually completely ignored).
Most recently, the series has taken a surprising ahistorical swing towards fantasy. Although the series has always played fast and loose with history - it's far more "inspired by" the historical events than setting out to painstakingly recreate them - the move into the Warhammer fantasy universe from Games Workshop was unexpected. It's also been an immense success, with both Warhammer games selling excellently and getting superb reviews. Total War: Warhammer II was released a few weeks ago to immense acclaim and the Mortal Empires expansion (which merges the strategy maps from Warhammer I and II into one super-campaign) was released this week.
CA have set out reassure fans of their historical games that they are being catered to as well, with three historical warfare games in development. CA's development slate currently looks like this:
Untitled Expansion for Previous Game
This should be formally announced shortly, but CA are working on a further expansion for the "biggest historical game they have ever done", one of their older titles. As well as featuring a new campaign, this expansion will update and revise the base game as well, possibly with better graphics and features. Although some fans are speculating this is a further expansion for Rome II or Shogun II, it may be an update and expansion for Empire: Total War. Released in 2009, Empire was the first game to use the current Warscape Engine and was infamously bugged on release (and some bugs remain in current versions of the game). Fixing Empire's remaining problems and upgrading the graphics would be a welcome move, as would adding a more focused campaign like the title's stand-alone Napoleon spin-off. This will be the next Total War game, likely to be released in 2018.
CA's Warhammer series was always going to be a trilogy and the third game will likely expand the map eastwards. Among the most likely new factions are the Chaos Dwarves, Ogre Kingdoms, Tomb Kings and Daemons of Chaos. It's possible this game will expand on the forces of Chaos in general, and may even incorporate the End Times events (presumably with the player having to the option to change the outcome, otherwise the game would be a major downer). Given the eighteen-month wait between Warhammer and Warhammer II, the earliest we can expect this is probably mid-2019.
Total War Saga
CA are working on a series of "smaller-focused" games like Napoleon and Attila, with the banner title of Total War Sagas. These games will still be Total War games but will have a relatively narrow focus. The first game in this sequence will be set in Ireland (CA begrudgingly admitted it after their "obscure" screenshot turned out to be readily identifiable) and will likely revolve around both internal Irish clan warfare and the arrival of Viking invaders. I'd expect this game in 2019.
Untitled Next Full-On Historical Game
CA have confirmed that the next historical game is in full development and they are working on urban battles (after complaints about the pared-down and underwhelming sieges in the Warhammer games). Although they have decided, after years of prevaricating, they will do further sequels in the future (making a Medieval III near-inevitable for the mid-2020s), this game will be set in an all-new era that the series has not visited before. I wouldn't expect to see this game until 2020.
As for the possibilities, the most likely one is China. The base game could be set in the Warring States period and DLC could then add in the Three Kingdoms, An Lushan and Jin-Song periods, as well as potentially the Mongol invasion, conflicts with ancient Korea and later periods. China has been completely left untouched by the Total War franchise, which is a bit odd given its suitability to the franchise, so this seems the most likely possibility. India and the South and Central American empires are possible, but some of the other ideas mooted (Ancient Greece and Persia, the Wars of the Roses) overlap heavily with already-published Total War games and don't match the "whole new setting" description.
An additional possibility, after the fantasy team has finished with the Warhammer trilogy, is that there will be more fantasy games. Strong candidates for this treatment would be, of course, A Song of Ice and Fire/Game of Thrones and J.R.R. Tolkien's Middle-earth mythos. There are already excellent mods for Medieval II (the last game in the series to allow in-depth mods) for these settings, but seeing official versions would be quite intriguing.