The next Total War Saga game is Troy, according to a trademark filing.
Over on the Total War subreddit, user zhxws posted a link to the listing on the UK's Intellectual Property Office website. Total War Saga: Troy is the name of the game, and the trademark is owned by developer The Creative Assembly.
Total War Saga titles are like the main Total War strategy games, but focused on a particular time period as opposed to an era. The first Total War Saga title, Thrones of Britannia, came out in May 2018, and was set in the British Isles, 878 AD, with 10 playable factions.