There's many ideas in blockland that nobody has ever used - This thread will bring some together.
Non-player boss battle
All of the current boss battle servers have a PLAYER as the boss - This concept has an evented build beating the stuff out of you instead. I think one guy did a server like this, but I haven't seen it yonks.
Giant monsters
All these loving mini cities and mini empires, and nobody has tried to beat the stuff out of eachother in it as a giant monster. Ever played War of the Monsters or Rampage? Maybe even that one Godzilla game? It's that. (i'm probably going to try this one out)
Arcade racer
Yeah, racing has been done before. But has anyone put consumable weapons into it, a la Mario kart or WipEout?
Motherloving Mech TDM
It's a TDM focused around Racer's old mechs. What's not to like?
Post some servers you'd like to see.