There should definitely be a regular chat warning, maybe even cords.
With the nether release, we could redo center isle into the end. Atop End Mountain (Someone else be creative I cant right now) sits the ender throne. When a Dragon spawns, he will circle the island waiting for adventurers to try and kill him. If no one does he ventures to randomly selected locations to attack the land. Any land he breaks will regen over time (Similar to TnT).
The ender dragon is the strongest possible fight on the server. Will require a party of several high level players and a decent composition (Not the extent that 10v10s can require, but will require multiple classes) upon killing it a slew of rewards are dropped. Everything from Souls, Relics, possibly even Codexs to dirt, tissues of tiers, or those "you tired" cakes you can get from crates/purchasing stuff.
Rewards can go up/down based upon frequency of the spawn, the difficulty of the dragon, and how much crap is felt appropriate to give away.
Also no idea if this is possible. So yeah. There's that.