To a large amount of games running on Linux on Steam, most should run on Opengl
Google focusing on Vulkan for new games is important, but it should give the possibility to run opengl for older games, in which the developers do not want to change the code too much! or have an opengl wrapper for vulkan to make as much old linux games running on Stadia as possible!
Steam has a huge list of games running on Linux and some old ones are good:
https://store.steampowered.com/linux#p=0&tab=TopSellers
Opengl to Vulkan wrappers:
https://www.collabora.com/news-and-blog/blog/2018/10/31/introducing-zink-opengl-implementation-vulka...
https://gpuopen.com/v-ez-brings-easy-mode-vulkan/
some examples of games:
Mad Max
Dirt 4
Dying Light
PayDay 2
Shadow Warrior
Middle-earth ™: Shadow of Mordor
Magicka 2
The Long Dark
Spec Ops - The Line
Xcom 2
Bio Shock Infinity
Life is Strange
Saint Row 3 and 4
Dead Cells
Stranded Deep
VICTOR VRAN
Aragami
Limbo
STAR WARS ™ Knights of the Old Republic ™ II
Deus Ex: Mankind Divided
Alien: Isolation
Bastion
Transistor
and many others
Another important thing is to make the Unity 3D and Unreal Engines fully compatible with Stadia! so that a game made on these engines is automatically compatible with the platform.