Other: Modified natural occurring 1 v 1 - ie, both sides have to accept the pvp in some fashion before the fight is really "on"
(I like Fallout's model - you offer pvp by attacking someone (for micro damage), and they accept by attacking back (for micro damage) and then the fight begins. if the other side ain't interested, you can try to get the kill with micro damage, but it's a) unlikely, b) seriously unrewarding (even if you kill them, you get no loot or money, a miniscule amount of experience, and you get slapped with the killer tag, which makes you appear on everyone's radar, and auto-accepts all pvp attempts with everyone that offers, for...3 days, I think? aka, anyone can "start" a duel with you by pot-shotting you for full damage. conversely, they encourage pvp by making it an exceptional way to gain experience, loot, and resources, if both sides accept - more experience than a similarly leveled critter, significantly more loot and money than is possible for any monster to drop. It's basically the best thing ever, if everyone wants to play, but it discourages some of the more bullying tactics that you can see in some games)
I also enjoy arranged huge messes - 100+ characters just going all out all over the place, either teamed or battle royale - something I'm looking forward to in the eclipse raids and sieges)