I'll repost something a bit tongue-in-cheek I put in another thread, though there's a serious element to it :
Maybe instead of the insane crazy buffs for mobs during DA, it would be less onerous to debuff the player characters in instances where they trigger it ? Lower movement and attack speeds, increased cooldowns after spell casting, penalties to mêlée, ranged, spell power and so on ?
Wouldn't change much in favour-farming by very over-level characters, but it would penalise the deliberate or careless gathering of huge groups of mobs at level in a less artificial manner -- just "explain" it by saying there's so many mobs in the encounter that it takes the DM much longer to do all his dice rolls, as a tabletop reference to what actually happens in D&D in these sorts of circumstances ... individual combat rounds can get painfully slow to play through. Maybe change the name from "Dungeon Alert" to "DM Overtaxed" or something ?