According to the latest blog by Michael Whelan, the CA team will revisit previous historical titles before releasing the next one. The important bit:
"Specifically, we have recently been looking at ways in which we might get back to historical content before we release the next major historical Total War, in particular thinking about additional content or standalones for some of our more recent historical titles. Jack Lusted and veteran team members from the ROME II and ATTILA teams have formed a new Total War team to investigate further, and we’re excited to share news of their intentions as soon as we’re able."
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