Nostalgia for Minecraft
A multiplayer persistent world like Minecraft with physics would be amazing, and probably replicate Minecraft's success.<p>e.g. If I chop down a tree at the top of a hill it could roll down and potentially wreck your house.<p>Or causing cave-ins and avalanches from indiscriminate mining. Building dams and rerouting rivers. Building realistic defenses and then siege weapons. Fun<p>Add in critter breeding and predation for dynamic populations that interact with each other. So if you slay too many Gorgs soon there would be none left. Leave them alone and they'll eat all the Fraggles. Players would form culling expeditions every week. Pretty soon you'll have players giving each other quests. :)<p>I think the venerable Ultima Online had a few features like that, or was supposed to, not completely sure.