So, we come to the third type of MMO I'd like to propose, PvP based. There are a lot of ways you could do a PvP based MMO. You could have a game similar to the leveling/exploration based MMO where it's all about world PvP (like Darkfall or EVE Online) where players have factions and guilds that are constantly in aggressive territorial disputes. You could have a PvP MMO similar to the Raid Based model where you have a hub town and join battle grounds and arenas (much like current PvP in WoW). However, the idea I had takes aspects of those and turns them into a PvP extravaganza. I think a cool take on PvP might be to combine all the PvP elements, while PvP-izing other MMO tropes, and creating what I would call Gladiatorial Role Play.
Imagine a WoW take on ancient roman gladiators. Your character is a merchant/businessman/noble who has decided to invest in the war games, as there is a lot of money to be made. The game begins when you pick what caste of society your character belongs to (each giving different bonuses, of course), and head off to the local orphanage. From the kids there, you adopt (buy) a young orcling boy, let's say (Achievement Unlocked: Brangelina - You have adopted a 3rd world orphan!). You take your orc home to your modest house that has an exercise yard and start training him to fight in the games. There are a few different starting classes to chose from for the lower echelon of the competition (grunt, brawler, adept) which you pick from the start which, as your ward progresses through the PvP ranks can branch out into multiple fighting classes (a grunt could advance to warrior or death knight, for example based on how you chose to train, where as an adept could become a mage or priest).
The way I imagine combat working is similar to WoW PvP now where you can have small 1v1/2v2/3v3/etc . . battles as well as battlegrounds types with up to 40v40. When you enter your ward in a battle ground, you take control of the character. Outside of instances, you control the sponsor. The sponsor's role will be to operate whatever business you chose (which could involve corporate PvP via hostile takeovers, bidding for resources) and to provide equipment and support for his fighters (an opportunity for auction house PvP in the form of aggressive bidding, buy low, sell high).
After a while, you could expand your compound and start training other warriors to fight for you (either more orphans or the more expensive mercenaries/adult slaves (too risky?). You could hire trainers and get better equipment for your fighters and become more affluent in the town. There could also be chariot races once you got enough to get a stable together (chariot drawn by war-bears, anyone?), and there could be a mode of play where merchants risk moving their inventory from one capital city to another via an escort mission through an instance. You could pay for extra insurance in the form of NPC guards, but you could be raided by other players.
This could stand to be fleshed out a little more, but I was excited about the idea when I thought it up and I hope it comes across as something with potential. I'd play that game.