UPDATE – It has been confirmed by Major Nelson that the new 1st party games will be added permanently to the Game Pass. So for those of you worried they would only be up for a month or so, you can rest easy.
In what I would consider a surprise announcement Microsoft have stated that 1st party games, such as Sea of Thieves, State of Decay 2 and Crackdown 3 will be available from launch on the Xbox Game Pass.
Personally, this makes the Xbox Game Pass much more attractive, and you get a discount for buying the full game too when you are subscribed. I am also hoping when Halo 6 and Forza Horizon 4 are announced they will be part of the Xbox Game Pass too.