As reported by Wowhead, the ongoing beta for the next WoW expansion, Warlords of Draenor, contains several tributes to Williams. This includes a lamp that can be activated to summon a genie named Robin who says, "Infinite cosmic power!" followed by "Itty bitty living space" -- references to his role as the Genie in Aladdin. If you have beta access, you can find the lamp for yourself on an island located near Talador.
Another tribute is located nearby, where you'll find a broken egg with a light and other things inside, a reference to the egg-looking spaceship used by the aliens Williams played on Mork & Mindy. Wowhead reports an alternate version of Robin's character that is dressed in a way that bears some resemblance to Mork exists, but is not currently in the game.
Finally, there are some toys scattered on the ground that been interpreted as a possible reference to the movie Toys, and another version of the Robin NPC that is dressed like Mrs. Doubtfire. Like Mork, this character (pictured below) can't currently be found in the game world.
Williams passed away on August 11 of an apparent suicide. He was a fan of video games (his daughter is named Zelda), including World of Warcraft, hence the online petition (and Blizzard's subsequent decision) to memorialize him in-game. Warlords of Draenor launches on November 13.