Interesting idea. A new PvP zone with a purpose.
The problem as I see it is that the heroes get a tough job. They need to kill the Giant monster while defending against player villains attacks.
The villains just have to fight the heroes to make sure they can't defeat the GM. To me this puts the heroes at a disavantage.
But a thing like this sounds like a fun special event.
Good point.
It could be remedied by having Longbow or signature heroes spawn on the map as heroes as well to stop the villains. That way the villains have to worry about something else, and it could also be used to balance the event. If more villains enter the zone, then more signature heroes spawn too to keep things balanced.
The opposite could also be used: if too many heroes enter the zone, then some signature villains could spawn too. But considering the heroes/villains ratio I don't think this would be a problem very often. But it still would be nice with an self-balancing system.
Still @Shadow Kitty
"I became Archvillain before Statesman nerfed himself!"
"HALT, I AM REPTAR!"
/signed 
This game needs giant monsters. And not just any giant monsters, but giant monsters that stomp!
So here's an idea: by doing a particular mission, a monster zone is spawned. It should be hard to spawn, maybe as the end of a strike force or even task force, much like Lady Grey task force spawns a Rikti counterstrike. Once spawned, the rewards should be high, so that people want to join, but not so hight that it's a farm thing.
The monster zone is a bit like the mayhem missions, but with four distinct differences:
1) It has a giant monster in it.
2) Everything, including buildings, is stompable.
3) It's randomly generated (heck, as we need new code to stomp buildings anyway, we might as well generate the map too).
4) Villains and heroes may enter that zone.
It should be easy to get to the monster zone. Any base teleporter, train, helicopter and ferry should work as an entry to the monster zone. It should be announced much like the monster events, so that everyone is aware of the impending doom!
The monster's objective is something on the map. It could be a warehouse full och tech stuff (for a Paladin), a graveyard full of dead people (Adamastor), a supertanker (Lusca) or just a place to nest (general Zilla thing). The monster zone ends when the monster reaches its objective, or if the monster is defeated.
The objective of the heroes is, of course, to stop the monster. The objective of the villains is to help the monster reaching its objective. Secondary objectives for villains are to cause as much destruction as possible, either by causing destruction themselves or by stopping the heroes stopping the monster. For heroes, secondary objectives are to prevent mindless destruction.
The monster reacts to things happening around it. If heroes cause a lot of damage if it goes one way, the monster could take another way, preferably through a building. But it's always trying to reach its objective.
The rewards are for the monster-related stuff only. Heroes get XP and/or inf for hurting the monster (awarded per team hit), or defeating the mobs that it spawns. Villains get XP and/or inf for stuff that the monster, the spawned mobs or themselves stomp. Heroes gets a massive bonus when the monster is defeated, villains gets a massive bonus if the monster reaches its objective. There are no rewards for PvP defeats. It only helps the monster or stops the villains from helping the monster.
Being defeated in PvP yields no debt either. Being defeated by mobs or monster does, however. If you're defeated, there's a spawn point on the map as per safeguards, so that you can get back into action.
The idea is that it should be furious, it should be fun, heroes should feel heroic and villains should feel villainous, and above all, Paragon City gets stomped!
Still @Shadow Kitty
"I became Archvillain before Statesman nerfed himself!"