It's kinda sad how games don't really work this way anymore. There are few open-world sandbox games and mods aren't really a thing. It's "free-to-play" skinner box treadmills from here on out.
Minecraft is still huge, and moddable. Sure lots of people moved to Bedrock edition on Consoles/Win10 but plenty of people still play the Java version and develop mods, resource packs, shader packs, etc.
Racing Sims like Assetto Corsa have a decent sized modding community with people designing and coding new car models, modifications to the physics/handling, audio packs, tracks/maps, etc.
Flight Sims are the same, and lots of other Simulator games.
As for open world games... Skyrim, GTAV, Empyrion, Space Engineers, Terraria, Starbound, are all moddable and have thriving communities.
Modding is very much alive and well. I'd suggest just finding any of the above games that interest you and having a play with the mod system. I find it a very entertaining and interesting way to learn more about programming due to the specific constraints of each game.
reverse engineering is your friend. i've modded all sorts of stuff that never intended it... Full Tilt Poker back in the day, MechWarrior Online, Ultima Online, etc. Learning how to hook into a program's functions and extract real-time data without destroying the stack, properly calling game functions with your own arguments, and translating your patches to new client versions is all pretty interesting and gratifying IMO.
