Well Back in 1.5 the server i played used to have a huge pvp event, "Gladiator".
It was a Clan server, very similar to kingdoms, clans would ally, declare war and fight every sunday for the gladiator prize.
The prize would be awarded to every member of the winning Clan/allies, it could be be something like this:
The member of the winning clan with the most kills would get a "gladiator" tag and the other members could win some keys, money or even xp.
Adding something like that to Kingdoms would be a great way to give it a bigger purpouse, to have a war going on, ppl getting redy for a big Fight
Back then the event would happen every sunday, but it could be monthly or in different arenas every week.. When the event oppened ppl could warp for the pvp off arena and choose a spot to settle. Then the warp would close and death or victory were the only way out.
On the arena combat tag is on so if you Log off inside the arena you die.
ps (you bring your survival items to the event)