X Com2 has amazing mods. Most of the content available for Battletech is from mods by like 2 orders of magnitude.

I won't even play beat saber without the mods. Half Life Alex has full games released in the engine as mods too!

Some I've modded besides what you said (Bethesda RPGs would be like, 8 games or something for me) would be Cities Skylines, Deus Ex, Factorio, Stardew Valley, Mount & Blade, Terraria, S.T.A.L.K.E.R.: Shadow of Chernobyl, Stellaris, Oxygen Not Included, Project Zomboid, Darkest Dungeon, Kenshi, Battletech.

I played vanilla Stardew Valley once, and now I'm on my second go around, and I've installed a bunch of qol mods so that I don't have to keep checking the wiki and my collections to see if I should keep or sell this item I got because I don't know if I need it for an achievement, and now I have a notification that it's an NPC's birthday and I can just check to see if I own or am carrying an item they have. It just makes the game less stressful for a completionist like me.

Does Tabletop Simulator count? For board game people, mods are the whole point.

Project Zomboid. Lots of mods that add new stuff (cars, weapons) and quality of life improvements like "Has Been Read".

Skyrim VR. The game without mods is probably one of the worst VR games ever, but with mods it becomes one of the most fun VR games.

7 days to die. The base game is in an...interesting state.

Cataclysm: DDA comes with a shitton of mods bundled and is superfun

ARK: Survival Evolved. There are no corner pieces for sloped roofs without mods... I like a good sloped roof, and a building that's not just a square/a-frame.

Sins of a Solar Empire: Rebellion has a lot of love and care put into mods. Being a sci-fi game, plenty of Star Trek / Star Wars / etc. conversions that do more than just reskin the models.

Minecraft is an obvious answer of course, but there are some other really good games out there that get made much better with mods. Some of my favourite examples of this would be Star Wars Battlefront II (2017) with the cool hero mods and bot mods, and then Ghost Recon Wildlands with the amazing First-Person mod.

While not "saved" by modding, Kerbal space program is certainly not as interesting.

Well, as you mentioned, Bethesda games in general. I always mod Elder Scrolls and Fallout games to a ridiculous degree. But some more:

Star Wars: Knights of the Old Republic has a great modding scene. I personally find "Start as a Jedi" essential for replays. And most of the mods (including that one) work on the excellent Android port.

Also, I love randomizers. I do a Symphony of the Night randomizer run at least 4 or 5 times a year, the Super Metroid x Link to the Past combined randomizer is amazing, and I'm finally going to dip my toe into Dark Souls randomizer soonish.

Not seeing anyone mention Crusader Kings yet (unless I'm blind)

Gameplay isn't for everyone but full conversion mods let you experience basically any world you want

Warcraft 3 and the little thing called DotA.

I don't know if improving is the right word, but the amount of transformative mods older games like Doom, Half-Life or Unreal Tournament (and not just shooters ofc) had, was wild.

Team Fortress started as a Quake mod, Counter-Strike as a HL mod, DotA as a Warcraft 3 mod.

Doom 1 and 2 would be a lot less interesting without mods, some of which change the game rather dramatically.

• Project Brutality turns it into a modern-ish shooter.
• Guncaster replaces the usual protagonist with a spell-casting, oversized-pistol-slinging dragon.
• DemonSteele replaces the protagonist with an anime character.
• My House and City of the Damned: Apocalypse turn it into a horror game.

Without mods, these games would have just been historical footnotes, not something a significant number of people still play.

Assetto Corsa. Outlived it's shelf life by many years, and holds up to racing sims with much more modern engines, all thanks to the incredibly dedicated modding community.

Fallout New Vegas for sure, although many of the issues were fixed by the devs over the lifespan of the game. Mods are pretty much vital to run that game properly, however. Many issues with the engine and many core bugs have been fixed by the community, and it's honestly something where once you play with them fixed, it's hard to go back

I already answered differently, but I want to put out that the STALKER fandom is held together by mods. There are everything from almost invisible bug mods to an entire standalone mod. STALKER is one of those franchises where modding just seems like such a natural fit to round out the world and it’s amazing how the vast majority of mods intend to support the tone of the game rather than just adding in the whims of the mod maker.