Why don't we have a mob arena with Pvp enabled? Make it a big, big area (was thinking .... toss in a few areas that are completely pitch black and a few areas near hazards.... You can choose to work with the other players or kill them. If the mobs die and there's more than one player, it continues until the players kill each other. Maybe the reward could be the gear they earn in there (You start off with no gear and earn it as the game progresses)
One idea would be an area much like the Cake Defense item drop zone.... Items will drop there at the end of each round IF a player is standing on a vulnerable pressure pad for a time period (10-30 seconds). In order to reach the drop zone, you'll need to navigate up to a very vulnerable area - Lots of mobs and easy to be shot by other players. The zone should also have a few hazards. Maybe it could be a narrow tower over lava with the way up on the outside.
The rest of the map would be mostly dark, claustrophobic and rough. It could be theme based (Stronghold?). Ceiling would be glass but far from a light source so that staff and spectators can watch progress. Mobs would be spawned using spawners interspersed throughout a series of rooms connected by corridors/caves. The corridors and caves could be fairly short (10-20 blocks) so you can't hide from mobs. The larger rooms could also contain some sort of bonus, like a beacon with a low level enchantment or a strategic point, so people would be drawn to the rooms. There would also be a few balconies overlooking some of the larger rooms. The balconies could be a point for participators to force other people out of the rooms. The balconies could be easily attacked from the remaining sides though. There could even be a second level, just to add some verticality.
The centre point would most likely be the item drop area, with large rooms in the corners of the structure and smaller rooms in between. I think this project could be easily built and be very popular... would require a low amount of people as the mobs won't spawn as much as Cake Defense. Probably 5+ players, with no upward limit. I'd be happy to build it myself if I get permission to.
Please post comments and thoughts below ^^
EDIT: When there is only one player left, they'll have to complete all rounds. This will be significantly easier than CD but there are some ways you can make it challenging. The final rounds might include tough mobs like blazes, witches or wither skeletons if they are approved. Remember, the gear is not enchanted and there are no potions. Just golden apples.
I also looked into Sethblings' hyper spawners and thought that you might want to make some zombies spawn with diamond armor and with some potion effects... that'd be simple to do and increase the difficulty. I was worried that players might hide when they are the last surviver to regen health between fighting monster. Just make it so that they can't hide without attracting mobs IE spawners or hazards every 24 blocks.
