Caves of Qud has nearly infinite replay value with all the random generations + mods. You can do nearly anything you set your mind to. Same with Project Ozone 3 (Minecraft mod). If you like goal orientated games that are multiplayer try Space Station 13. That shit is nuts, and I mean 13 not the 14 on Steam it doesnt have half of what the old one does. But the launcher is about to go out for 13 so get it while you can.

I found that Two Worlds had a lot of fuck around potential. You can infinitely combine weapons that are duplicates to strengthen them. I once killed every NPC in the entire game and used the resurrection spell on them so everyone was completely loyal to me even monsters.

If you can still find it. There was this online Pokémon game that was somehow realistic. You controlled your character typed ccommands for your Pokémon. And they were always alongside you when out of their ball. It was hardcore as shit. I got my cheeks clapped multiple tines trying to run to a different village by gangs of mankeys and caterpies. Nintendo is just drooling on the floor not doing this officially. It took forever to level, it was an extreme challenge to find stuff to evolve your Pokémon. Ugh I hope thats still out there somewhere.

A less spoken one nowadays that is also very fun is still the original Morrowind. Absolutely mind blowing how expansive that game is to this day. It is still just as legendary as it was back then even if it is a little dated. I actually come to like it when a game isn't guiding me on what to do in an RPG. It feels more immersive when I'm supposed to know what a Guar looks like.