You have my +1. I would say to scale it to where you need 1 healer, 1 dps, and 1 tank. Give the 4th regular spot to a gather/crafter and make the waves more dependent on gather's/crafters the longer it goes. Just a idea here, make every 5 minutes a goal where gather's/crafters need to gather and make X amount of items. For the combat classes, every 5 minutes they need to defeat 50-70 mobs which is not unreasonable if scaled correctly. Make it a set ilvl as well of around 300ish.