If we could get everyone into a game of The Ship that would be a great time XD
I guess TF2 falls into the category of something most everybody could play since it's free, granted everyone can run it at a decent speed.
Minecraft or Terraria could be fun and offer enough creative freedom that we could actually make some cool Delver's Drop-related stuff, maybe even to post on the forums!
Plain Sight's another game with a kind of dead community, but it's a legitimately fun and unique multiplayer game, so I'll throw it out there.
I'll keep it to those for now, but yeah, I'd be up to participate in something like this if we could arrange it!