With Lord of the Rings back in the zeitgeist, courtesy of the new Second Age-set TV show from Amazon Prime, it seems a good time to revisit the concept and with more money and the success of two (soon to be three) Warhammer games behind them, Creative Assembly can make a good case for acquiring the licence. The only fly in the ointment might be that the licence is currently held with Warner Brothers' gaming division (who produced Shadows of Mordor and Shadows of War) and it is unclear if they would be willing to sub-licence the property to another studio.
CA also note that they have been in discussions with Netflix over a new project. Since they've already effectively made The Last Kingdom: Total War with Thrones of Britannia and we can rule out Unbreakable Kimmy Schmidt: Total War, the most logical property to pursue would be The Witcher, which has all the ingredients for a good Total War game: numerous factions with notable troop types and general characters fighting over a fantasy landscape. CD Projekt Red, rather obviously, has a Witcher video game licence already, so the viability of this idea depends a lot on what is permitted under that licence, whether CA would work with CDPR or whether a separate licence based on the TV show (rather than the novels, which CDPR's licence covers) is possible.
Since CA note they have been in discussions with Amazon Prime, that is likely related to the Lord of the Rings idea, but it's also possible they have been considering a Wheel of Time game. The Wheel of Time world has numerous countries, empires and factions with notable characters who could serve as generals. A Wheel of Time fan mod for Rome: Total War was unfortunately aborted in the mid-2000s over the inability to get flying creatures to work in that engine, but with modern tech, that shouldn't be a problem (and flying units play a major role in the Warhammer games).
A final possibility, which is unmentioned in the documentary but has almost certainly been discussed at CA, would be for a Game of Thrones game. There is already a popular Westeros: Total War mod based on Medieval II: Total War and the success of the TV show makes a game a bit of a home run. Given the Total War franchise is all about changing history, the game would also provide the possibility for different outcomes to the TV show's storyline, which I'm sure many fans would appreciate. If you always wanted to see Robb Stark triumph and win independence for the North, or Stannis Baratheon to win at the Battle of the Blackwater, you could in such a game.
Any announcement of a new fantasy project is likely a way off. Total War Saga: Troy launches in August, whilst the company is continuing to release new material and expansions for Total War: Three Kingdoms. Total War: Warhammer III is also due in the near future, so it might be a year or two before we hear about any post-Warhammer fantasy projects.