Having played in the mob arena, (SO MUCH FUN) it felt a little "cramped" maybe make it a huge complex. Give more space the the contestants, and you'll be able to spawn more mobs as well.
Also make it a kind of "round" thing. Like a
Zombies only round in round 1,
round 2 LOTS of zombies
round 3 skele's,
round 4 skele's and zombies etc etc.
Something like that maybe. Some feedback would be great.