Graal Online, Maplestory, Terraria, Starbound, Minecraft (there are a few clones already), Rust, CS:GO, Super Smash Bros, Splatoon, Animal Crossing, Rockman.exe, maybe something like Prototype or Ultimate Spider-Man with fun physics so you can swing around and do whatever.
A free recreation of Graal Online would be especially cool as the content was all player-made with the available development tools. They just let the PC version of the game wither and die. If someone just made a solid base, a community could form to make the actual stuff to do. I've heard people compare Graal to BYOND/SS13, although I never experienced those. There's SS14 now, but it seems pretty focused on... space station stuff. I don't know that it's similar enough to Graal for me.