A multiplayer persistent world like Minecraft with physics would be amazing, and probably replicate Minecraft's success. e.g. If I chop down a tree at the top of a hill it could roll down and potentially wreck your house. Or causing cave-ins and avalanches from indiscriminate mining. Building dams and rerouting rivers. Building realistic defenses and then siege weapons. Fun Add in critter breeding and predation for d…
There was a game or tech-demo called "upvoid miner" a few years ago. It had worldgeneration similar to minecraft and proper physics with rolling tree logs. It allowed arbitrarily shaped terrain and I think liquid physics were worked on. But sadly the studio had to shut down due to not getting enough sales. I think you can still download it and test out the free tech demo though.
GPLv3, unfortunately, but interesting nevertheless.