I used to run my own Ultima Online server with an emulator called RunUO about 15 years ago. The same sort of fun I had running that server is what I see people enjoying in Minecraft today. Its really great to see. It was possible back then with a .Net windows server to support 1000+ concurrent clients with very little noticeable lag.
I'm currently working on my own game in Unreal Engine 4 but if I could clone myself I think I would set about creating a Minecraft client that could connect to a RunUO server so that you could achieve those kind of 'MMO' numbers. As well, you could leverage the RunUO codebase to add support for a lot of the Ultima Online functionality (home ownership, spell casting, deep crafting, etc, etc)
What is the best community to look into for supporting massive numbers of concurrent players in Minecraft?