I am hoping that the upcoming "Challenge System" (ala Crystal Cove) implementation includes some group PvP challenges. Seems like a pretty open door to allow such things without the need to directly change the PvE game.
Group A vs Group B trying to achieve something in a short amount of time while defending against each other.
For example each group has to deal damage against certain "pillars" or some such and the group that deals the most damage "wins" the round. Sort of like Kill the Flag instead of capture the flag. Throw in some random MOBs that both groups would have to fight against to keep things interesting.
Then again I might be the only one who thought about PvP the minute I first stepped into Crystal Cove...(competing for crystals / protecting kobolds / etc) seemed like an obvious prototype to me. *shrug*