Currently the cadence for Pro games on Stadia seems to be two a month. Considering that the entire library currently consists of 25 games, that really isn't too bad. If players jumped on board upon release (and everybody did, because they had to), they would have six free games as of January 1st. Not too shabby.
Stadia announced today via Twitter and on the official Stadia blog that the new free games for January will be Rise of the Tomb Raider and Thumper. Both of these are great games, but the somewhat oppressive and terrifying rhythm game Thumper is really the killer app to pull out when you have guests that are questioning Stadia's lack of controller latency. Thumper is almost certainly in the library simply to prove Stadia's functionality to doubters.
The blog post goes on to mention that the original Tomb Raider will be leaving Pro on January 1st, so be sure to grab it sometime this week before it disappears. Once claimed, a game remains in your library for as long as you maintain your Pro membership. If you let your membership lapse, your Pro games will be waiting for you when you sign back up.
Stadia has quickly become my platform of choice since release. I'm having a great time with the system, and am looking forward to bringing you more Stadia news as it becomes available.