I don't know if you're making a fantasy arena style or a Roman gladiator style map, so not everything may fit with each style.
-The gladiators work together to fight a great boss, the Juggernaut. He drops an item which grants great damage, but is dropped again on death. The gladiators then fight in a FFA battle for control of the hammer. In the end, there's one person alive, and he gets the hammer to keep.
-A crowd of humans enter the arena, but they are infected and quickly turn into zombies. The gladiators work together to defeat the zombies, but each hero that is killed turns into a zombie himself. Ends when all zombies or gladiators are dead.
-Catapults are placed on the sides and a large group of enemies enters the arena. The catapults fire on the gladiators, but deal damage to both gladiators and enemies. (the Gladiators can use this to their advantage) The catapults should kill a lightly armored gladiator in 3 shots, and a heavily armored gladiator in about 5. (AKA, lots of damage)
-The gladiators each enter a small arena and need to kill a certain amount of enemies by themselves as quickly as possible. (no interaction between gladiators, more of a competition)
-Hidden landmines (bombs) are hidden around a track. The heroes then race to the end of the track without being blown up.
Good luck on your map!