Terraria. It’s a sort of hybrid between a minecraft 2d and an action RPG, really fun game with lots of content.

You can play it as standalone or as co-op and the server component is included in the game for you to host it if you want.

Tho If by interesting you mean MMOs, there are emulators for a few of them, WoW is my favorite (TrinityCore, AzerothCore), but know that opening them up to public use is illegal (copyright violation).

Edit: added github links to wow emulators.