Large fancy stone in a prominent place in a level 85 zone. It takes 10 seconds to channel a cast on it that can be interrupted by any attack. A large radius around this stone is a FFA pvp zone. The person who successfully casts on the stone gets the stone as a trinket.
Once they have this trinket they cannot fly and they drop it if they log out. In addition everyone’s map shows where they are at all times. The trinket gives them a massive power boost but they lose their faction while they have it – anyone can attack them and they can attack anyone (but they cannot stealth). If they drop it others can pick it up, but once it’s been dropped it has to be picked up within X amount of time or it will teleport back to the zone it starts in (and be unobtainable for a small amount of time to give people a chance to pile back over there).
It could come once a week for a day or for whatever amount of time and frequency prevented this from becoming annoying and getting old too fast.
If additional rewards want to be added there could be a quest that the trinket itself gives you which asks you to make it to a certain zone (remembering you can’t fly and you’ll be hunted) to get a reward.
EDIT: forgot an important point - you can't take it into raids or BGs. The point is not for a guild to get it and go kill bosses with it...the point is to cause a massive FFA with the reward just being the fun of charging around very powerful and knowing you are likely to call lots of max level players to you where ever you go...
If more rewards are needed they could be added though...but since the entire point is world PVP rewards should only be added if people aren't having FFAs over it already...