Since 1.4 people have wanted to fight the Wither boss on the server, but not many people have said much about it on the forum so I did a little thinking and why don't we make a arena out of bedrock? It would protect the land around it, so no mob griefing, and you could put in the sky for extra safety.
Fighting the wither will be at your own risk, so if you die, game over but if you beat it you get the Nether star, witch you use to make a Beacon. Beacons are a high on a griefer's list so if people set it up it will be at there own risk and most will get sponges for protection. Some might think the Beacons are OP but if you see how hard it is to get it and to get all the minerals it takes, its kind of worth it.
The lag is my only concern most servers don't lag much but there tends to be a drop in frame rate.
I hope this goes into consideration. Have a jolly good day.