Flathub (flathub.org) actually has quite a few good games. Flathub is a collection of Flatpak applications and games. It's kind of like an app store. Flatpak is used to distribute software across multiple GNU/Linux distributions, without dependency issues or having to use distribution repositories with outdated software.
Most of the games over at Flathub are open source games, but some are proprietary. The games below are a mix between those two.
0 A.D. – A real-time strategy (RTS) game of ancient warfare. It has been in alpha for a long time now, but you should give it a try, if you like these kind of games.
Albion Online – A sandbox MMORPG with a top down perspective. It has a fully player-driven economy, full loot PvP, and a classless system ("you are what you wear"). Albion Online also has dungeons, gathering, crafting, and so on. Read more about Albion Online in a previous blog post I wrote.
Battle for Wesnoth – A turn-based strategy game with a high fantasy theme. This game is really good.
OpenTTD – An open source simulation game based on "Transport Tycoon Deluxe". I love the old school graphics in this game.
RuneScape – A high fantasy open world MMORPG. You can spend a fair amount of hours in this game.
RuneLite – An open source client for Old School RuneScape (OSRS). RuneScape, but the old school version of the game.
Shattered Pixel Dungeon – A roguelike RPG, with pixel art graphics. It has randomized levels and enemies, which give it some degree of replayability.
Veloren – A multiplayer voxel RPG. It takes inspiration from games such as Cube World, Minecraft and Dwarf Fortress. The game is currently under active development. I felt it was a bit empty and lacked a direction for what to do when I tried it, but the potential is there.
All images taken from Flathub, except for the Albion Online one, which is my own image.