Roblox is designed for MMO play from the start, but worlds (or whatever they're called) are completely open to modification with their free editor. Then you can script things in Lua. Lua!
The other night my son was antsy at the dinner table. So I asked him why the rush. He said he "had a flight to catch". Turns out he's joined a group that operates an "airplane" that has scheduled flights in and out of an airport that someone has created. He interviewed to become the first officer on the flight.
It's messier than Minecraft, but the players seem to understand the potential. They're hiring, actually. (no I don't work for them). The stats on this page are interesting too: