The Long Dark is brutally cold and brutally challenging

Project zomboid

Days Gone is a great open world game, with motorcycles and zombies.

Stalker has sandbox elements. Dwarf Fortress probably, but it’s highly addictive at least for me. Darkwood is pretty good. Core Keeper, Minecraft, Raft? Outward… Terraria…

Terraria

If you want a no-combat platformer/puzzler, Sable is a good time with beautiful art.

If you want a hardcore RPG sandbox, try Kenshi.

Icarus if you want an FPS Terraria for side scroller OpenTTD for retro top-down Cities Skylines (sequel out next month) for city sim

I’d you’ve never played Kerbal Space Program (the first one), it’s got a Linux native version and a mod manager specifically for Linux. It’s probably the best sandbox I’ve ever played (2000+ hours).

BeamNG Drive. I have way too much time in that game and I have barely touched making mods.

No Man’s Sky (Proton).