As seen on a link shared on a post by Reddit user zhxws, the next Total War Saga could be Troy.
The link, posted on the Total War subreddit, reveals the trademark “Total War Saga: Troy,” which appears on a page on the UK’s Intellectual Property Office website. The trademark was filed on July 23 by owner The Creative Assembly.
Unfortunately, the page shows no description of the upcoming game itself, but with Gamescom coming soon, the game may be officially revealed there.
[Source]: EuroGamer.net: It looks like the next Total War Saga game is Troy.