"Basically, fast-paced interesting ADHD gameplay. Combination of Dota 2, Team Fortress 2, Overwatch, Valorant, Smite, Orcs Must Die."
That's how notable Valve leaker "Gabe Follower" describes Deadlock, a Valve game that is seemingly in playtesting at the moment, for which a few screenshots have leaked out.
The game has been known as "Neon Prime" and "Citadel" at prior points. It's a "Competitive third-person hero-based shooter," with six-on-six battles across a map with four "lanes." That allows for some of the "Tower defense mechanics" mentioned by Gabe Follower, along with "fast travel using floating rails, similar to Bioshock Infinite." The maps reference a "modern steampunk European city (little bit like Half-Life)," after "bad feedback" about a sci-fi theme pushed the development team toward fantasy.
There are similarities, but Overwatch bears way more similarity to a MOBA with a wide variety of heroes using hard coded interactions rather than TF2 which uses a smaller selection of classes with variety within the class, who usually have some degree of shared systems and interactions.
TF2 still gets new player tech discoveries due to the design being more open in the classes interacting with the map and each other. The issue kind of is that this is due to TF2 being "antiquated" in having not been designed for competitive professional play, and having been essentially put on maintenance mode, so there are less devs interested in going in and "fixing" things.