Gaming

Cataclysm Dark Days Ahead

https://cataclysmdda.org/

Zombie survival roguelike. It's free, open source, and a mature project.

Closest I can think of to infinitely replayable games are rougelike games like Slay the Spire, Peglin, Shotgun King: The Final Checkmate, Backpack Hero, etcetera, and sandbox games like minecraft (or Minetest if you don't feel like spending money and/or don't already have a minecraft account).

Though, with the rougelike games I mentioned, there are upper limits to increasing difficulty levels.

Kerbal Space Program, for sure. Skyrim is another.

How about a randomizer? Pick a game you like and see if there's a randomizer for it:

https://video-game-randomizers.github.io/rando-list/

Fallout: New Vegas. Skyrim. Especially with alternative start mods.

Anything with procedurally generated levels, like roguelikes/roguelites. I can personally vouch for the longevity of Slay the Spire, for one specific example.

Fallout: New Vegas, Caves of Qud, Project Zomboid, Minecraft, Terraria, Morrowind, Skyrim, Dwarf Fortress, Kenshi, Rimworld, Elden Ring, and so much more.

Many ppl suggest rougelike/lite and sandbox games I want to also add games with a good mod community and have a lot of side quest like Skyrim, they fit the replayability criteria because when it starts to feel the same can you add mods that change things up. I have done thousands of hours in Skyrim and never finished the main quest 😂 I think Balders gate 3 will also live for a long time. Many rpgs seem to get a lot of mods and games like Balders gate change a lot depending on what you do and how you play.

But you seem to want some kind of fps so warframe would be better, you can play alone or with friends. Just like ultrakill do you jump around killing, you can use, swords, guns and magic depending on your build (there are many). Doom would also fits your style of playing I think.

Otherwise management games is a time sinker. But most do not have fighting elements where you yourselves fight. Cult of the lamb tries to be all of it, it has action/fighting and management you can even decorate if you are into that. Pretty good for those who want it all. The devs still updates it too.

The binding of Isaac.

I've been playing it on and of since the flash game released 13 years ago.

Deus Ex

Europa Universalis 4. And EU5 is on the horizon.

Definitely not like the games you've mentioned though.

Generally games with random elements are considered to be good for dumping tons of hours into. So games with randomly generated worlds like Minecraft, roguelikes, strategy games that are always variant just because of the nature of AI actions always being a little randomized, and other stuff like that. So maybe like Minecraft, Dwarf Fortress, Crusader Kings 2 or 3 as like a basic list. But really the game that's going to be the most replayable is the one you don't get tired of. I've beaten Thief: The Dark Project hundreds of times and that game is a relatively simple level-based stealth game with no random elements and not even especially huge levels.

I've been recently playing the "so called" sequel to ftl, ftl multiverse. I have gotten the standard and old secret endings, but there are hints of a third even harder to find ending.

Doom. No the originals. Doom, Doom 2, and Doom Final. I always go back every few years.

You can get hundreds of hours out of Binding of Isaac, easy

If you're into boomer shooters, you can't go past the original doom for infinite playability. Literally 30 years worth of user created content and mods.

War for the Overworld (I have many thousands of hours in this)

Settlers 2 Gold

(Both the above have a lot of user generated maps, which has given them most of their longevity.)

Factorio

Stellaris, Rimworld, the Sims, and a lot of stragedy games.

Shattered pixel dungeon

For me Rollercoaster Tycoon 1&2. But you could probably throw most Sim Games into here like SimCity 2k 3k or 4.

For me, Noita. I don't recommend it unconditionally, but for me that game will forever be the only permanent game in my library. I expect it's possible that I could finish Elden Ring. I know I will never finish Noita.

I put multiple thousand hours into Mount blade warband, Crusader Kings 2, Europa universalis 4, and more.

Diablo 1/2

Dynasty Warriors 2, Dynasty Warriors 3, Dynasty Warriors 4, Dynasty Warriors 5...

Stardew Valley

The one I never get tired of replaying is Mafia 2. Maybe a weird choice since it's very linear and every play through is pretty much the same. But the story is just perfect for me. The gameplay perfectly suits the story, nothing especially clever, just well executed.

Then there's games like Binding of Isaac or Dead Cells where every play through is different.

X4 foundations and relevant DLC. I have multiple saves with over a year worth of playtime

Factorio and RimWorld immediately come to mind. Even with the base unmodified game, you’ll likely get several weeks of gameplay out of Factorio. Then if you dive into modding, you’ll never put it down. Multiplayer is really nice too. And their big DLC just got announced, and is planned to drop in a few months. So now would actually be a great time to dive in, because you’ll get access to the DLC about the same time that you’ve burned through the content on the base game.