If you're interested in roll-your-own things like this, stop by #mcdevs on Freenode and take a look at our documentation at http://wiki.vg, where we've been RE'ing the protocol and game internals for 8+ years.
And wow, I can't believe Minecraft is 8 years old -- feels like I first played it just yesterday.
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?
I have no idea about what the players want these days. The large-player-count servers I believe still use instancing and transferring players between servers to support those numbers.
The other hard part is the game's mechanics. You'd need a lot of careful game design to reign in the destructible nature of the game world and all its contents. Minecraft isn't about questing and loot, it's about construction, and if you're not leveraging this you might be better served by making your own game. Large servers typically address this with round-based deathmatch or deathmatch-like gameplay, not really what you seem to be interested in.