You can implement the games and rewards for PvP here like in other games, but they won't be very good. You can get hundreds of players to fight, but the fighting will be unsatisfying. It won't become nuanced and strategic like Shadowbane just because you add 50 more players to the chaos.
Most GvG games allow players of all levels to at least contribute to GvG or PvP. In DDO, there are dramatic imbalances of power, where players frequently 1 shot one another. There's no limit on how many players a Web can snag or a Wail can kill (with a DC of 40, mind you). There's no way a cleric can heal damage from even 1 600DPS attacker. There's only 1 way a character with a +24 to-hit can hit a level DoS with a 65 AC: roll a 1. And so on.
Even if you had 100 versus 100 mass warfare, the quality wouldn't be good. Once people realize the quality isn't good, they won't do that, and will focus on improving their characters.