Can this be Crate’s next game engine?
https://godotengine.org/
It’s Open-Source, 100% free (donations are welcome to found this project), very powerful, easy to use and most importantly it supports Linux natively.
Their next game (the city builder) was already confirmed to be in Unity.
Missed opportunity. 
Meh, I’d personally prefer Cryengine or Unreal tbh.