Harmonix, Uppercut Games, and Supermassive Games are working with Google to create Stadia-exclusive content.
Harmonix is probably best known as the creator of the first two Guitar Hero games, but it's worked on many storied rhythm games over the years, including several Rock Band titles. Uppercut Games has several former BioShock developers on staff and has a few games under its belt, most recently Submerged and City of Brass. And Supermassive Games is known for branching narrative horror games like Until Dawn and Man of Medan.
All three studios are now part of the nebulous Only on Google Stadia umbrella, Google's effort to beef up the studio's exclusive offerings. With more than a dozen solid games coming to the platform this year - including Sekiro, the modern Hitman trilogy, and sports titles like NBA 2K21 - Stadia isn't lacking in third-party options, but it's still limited by its exclusives.
Harmonix, Uppercut, and Supermassive are capable and wildly different studios, so perhaps their creations will spice up the platform's library - whatever they are, and whenever they come out, that is. We don't even know what kinds of projects these studios are making just yet - they could be Stadia exclusives or just unique features for the platform - so there's no telling when we'll actually get to play them.
In the meantime, you can play another new Stadia exclusive, Crayta. Crayta has been pitched as a game maker that Stadia players can use to build and share multiplayer games, which makes sense given the structure of Stadia. Perhaps future exclusives will also seek to leverage the platform's always-on design and feature set.
Serious Sam 4 is also coming to Stadia (and PC) in August.