0Love Life of an Ooze: One ooze. Idiot hits ooze. Two oozes.
*insert axe*
o o
PvP quests in which parties fight each other is a bad idea, period. As everyone else has said, it can't be done without imbalancing PvE. And while games that manage a halfway decent balance between both exist, they were built that way from the ground up. This is not one of those games. Doing so would require you to alter the D&D system so thoroughly that it could no longer, in good faith, even be called DDO.
HOWEVER...
The idea of competetive quests, where one party competes with the other in some other way, is an absolutely AWESOME idea.
Maybe you're racing to be the first to kill a boss, or reach the center of a maze. Maybe you're trying to kill the most mobs within a certain time limit. Maybe you're guarding an NPC against a neverending stream of mobs, and the first party whose charge dies loses. (This one could even be balanced for offense, by allowing you to attack whatever their protecting yourself, but in doing so you'd leave your own charge unprotected, and the mobs attacking your opponent could aggro on you as well.) Maybe they could even just add an in-game setting that would track and compare your completion time for a dungeon with another party's automatically. Either way, even though the game isn't balanced for actual PvP combat, there are other ways to pit players against each other. I think that in a game that is based around players working together as a team to get past obstacles, being able to pit one team's ability to do so against another's would be quite fun.
Admittedly, you could do the same thing informally, simply by timing yourself and comparing it with people in general chat and on the forums, but you could do the same thing in a single player game. Might as well make use of the MMO format's ability to interact with numerous other people simultaneously, right? Besides, even if you're not actually fighting them, per se, being in the same dungeon with and actually seeing the party you're going up against would add to the sense of immersion quite a bit.
EvE online... As cheap as the game mechanics make it, any player, can do anything, anytime... All they need is a huge helping of friends (Depending on what they wanted to kill). and an understanding of game mechanics to be able to exploit **** near anything...
No class change, and everything's balanced.