Looks like Valve's next game is Deadlock, a multiplayer shooter-MOBA mix of Overwatch, Dota and Team Fortress

Valve's upcoming game, "Deadlock," a third-person hero shooter with MOBA elements, has leaked through screenshots from a closed playtest. The game combines aspects of Overwatch, Dota 2, and Team Fortress 2, featuring two teams of six players navigating large maps with unique hero characters. Originally known as Neon Prime, the game has shifted from a sci-fi to a fantasy theme after feedback, incorporating a European-inspired setting with steampunk and fantasy elements. A formal announcement of this "major" game is expected soon.