New characters have to fight monsters one or two at a time anyway, or they will get ripped to bits. Experienced characters are too powerful and can streak into town naked and bludgeon a hundred zombies (hulks included) with a piece of bubblewrap.
Could monsters be equipped with a stacking aura* that buffs other monsters (within, say, two squares) and gives them higher to-hit, higher damage, greater knockdown chance, damage resistance, etc? Could this be made apparent in an unobtrusive but clear way through the UI? For instance, by showing up in the eXamine [“This monster is in a group of nearby monsters, making him tougher and more dangerous.”] and in the event texts [“The Grabber Zombie lunges forward attempting to bite you! The attack is more dangerous because of the other nearby monsters!”]
I think this would be a reasonable and realistic way of keeping stock monsters dangerous for experienced characters, without making life much harder for new characters. If you meet one zombie on the street and you’re a master of machetejutsu you can just machete him in the face and get back to dropping acid and hoarding romance novels, but if there are thirty of them pressing in on you and you don’t have room to swing or dodge, they’ll still be snacking on your brains.
*For instance, if the bonus is +1 attack, +3 damage, +1% knockdown, +3 damage resistance, and there are 10 monsters within two squares of monster X, monster X gets +10 attack, +30 damage, +10% knockdown chance, and +30 damage resistance. If you think linear scaling is too much, maybe use a square-root or otherwise diminishing returns model.