I would like to mention Avorion. Its sandbox space game developed by two guys with heavy emphasis on user-created content and procedural generation that I've bought on Steam sale two years ago for 7EUR and devs are still updating it adding new features and fixing issues:

https://store.steampowered.com/app/445220/Avorion/

Its considerably small and limited in scope, yet game progression feels rewarding enough that some people (me included) have clocked hundreds of hours doing it.

This is the kind of game I feel we need more of. Small games focused on limited number of features, continously improving those features.