Openttd trains for ever

Endless Sky is a pretty cool open source space game.

Also Dwarf Fortress.

Kenshi. Sandbox rpg, citybuilder and more. Pretty unique and unforgiving, but the replayability is endless if you are into that sort of thing (lots and lots of mods as well).

I've played Stellaris for years, although I am really not liking the direction Paradox is going with their AI, and it really puts me off of their games.

Factorio

There is Luanti if you want something Minecraft like. It has a built in modding API and tons of mods available. It supports multiplayer over the LAN. It's also very lightweight, it will run on almost anything.

Do you know of Songs of Syx?

A bit like a large-scale rimworld, with a focus on managing a big population and world diplomacy/war.

A bit hard to get into, though.

Also, what about project zomboid? A bit far from your description I guess, but who knows.

Valheim has a local server you can set up that has it work offline. Shame it's not on gog, but it's on Humble which I don't believe has online checks like Steam, but I had it working with steam offline mode.

Starbound and Terraria come to mind. Also Planet Centauri and maybe Stardew Valley.

Also Minecraft if a DRM-free version other than the Pi Edition (which is abandoned) ever comes.

NetHack

Endless fun

Satisfactory, building game, a bit heavier on resources since it's 3d but it's good

Factorio is great, how has no one mentioned it yet? Also Dwarf Fortress, Into the breach and "FTL - Faster Than Light".

They all have native ports and are moddable.

Factorial and FTL even have huge overhaul mods, that change most stuff so they don't really get boring.

Minecraft: Java Edition. You need internet the first time, but afterwards it's fully playable offline and also has LAN support built in.

Vintage Story. It's a very good survival, very customizable experience too.

I think it's a game that shines best in a multiplayer civilization RP tbh but even as a single player game it's a very nice survival. It may look similar to Minecraft at first but it is very very different and in all the good ways.

Plus mod support is so good, by design, even the website itself has 1-click installs.

Oh and it has chiseling.

Caves of Qud. I's like if Dwarf Fortress' adventure mode was actually good, and set in hyper-post-apocalypse scifi world where the very water you need to drink (or if you're amphibian, splash on your skin) is also the world's currency

Maybe CDDA with some difficulty setting changes to reduce combat difficulties and push it more on survival/building.

Rimworld.

ICARUS. Works perfectly under proton.

The Planet Crafter seems pretty neat, though I've only played the demo. It's a first person survival crafting game where you terraform a planet.

Starbound! That game kicks ass.

Vntage Story is pretty neat, it does require a WAN connection to start it for the first time but AFAIK it's possible to play it completely offline afterwards.

Beimg as mechanically complex as it is you'd think you need a wiki open the entire time, but the ingame "guide" has everything you need to know.

I don't know about the state of public multiplayer servers, but it is LAN compatible and it actively supports modding!

Euro Truck Simulator 2

American Truck Simulator

I have thousands of hours in them 👀

Xcom 2: War of the chosen

If you like 4x games, I would suggest X3:Terran Conflict/Albion Prelude from Egosoft. Its older, so wont be a burden,(CONTROVERSIAL OPINION ALERT) and probably the last good game from egosoft (/CONTROVERSIAL OPINION ALERT)

Space game set in a huge world with multiple factions and in a breathing world that can and does change with time with or without your actions.. hundreds of hours of gameplay, with options for building your own empire.. and its got a huge modding scene that can make it even more alive.

Once you start getting your personal economy going, you can take a more macro chill approach to the game, you can sit in a mothership issuing fleet commands and letting the AI handle everything, or you can get right into the cockpit of a corvette/fighter and dogfight yourself. While building massive station complexes to produce goods to sell and generate funds with.. or to build your own ships and fleets with.

Technically, Satisfactory (3D Factorio, but more chill) allows you to host your own servers. You can host a server in your LAN, connect to server via IP -> enter the server's local IP.

There is also Terraria which is very low spec.

I recnetly started playing a game called "StarSector"

Basically Mount & Blade in space someone said.

Really good so far, have not played more than 30h due to lack of time to play due changed life/work situation.

Edit: forgot to say that it is Single player but tons of mods that feel very much in line with the game.