Games with lots of mods for sure. It may be a generic recommendation but Minecraft with mod packs and gorgeous shaders is an amazing experience for someone who never got around to trying them.
Also if he’s a souls-like fan, I highly recommend Elden Ring’s convergence mod and Dark Souls 3’s convergence mod (they’re by the same team). It adds like a hundred new spells, new weapons and equipment, overhaul of some mechanics like having actual classes with game-changing effects, completely overhauls areas with new geometry, adds new enemies and bosses, etc. It’s a very fun time.