Job ads for the new project appeared on the studio's careers page yesterday. They don't reveal much but we can glean a few things about the new IP from the positions Creative Assembly are hiring for - a senior meta game designer, lead systems designer, and a lead technical artist.
Creative Assembly want the meta game designer to create the framework for the game "economy, progression and retention methods, leaderboards, challenges, world building and events." Strongly suggesting the project is a service game with multiplayer.
The job description for the systems designer refers to "weapons, movement, abilities, gadgets, health and other underlying gameplay systems." Which aren't exactly unique to a particular genre, but to my mind abilities lend themselves to games that are more RPG-based in nature.
https://www.pcgamesn.com/creative-assembly-new-ip-hiring