Other than the occasional sneaky Antling Digger griefing unsuspecting noobs, I agree the mob distribution is very monotonous. Entire beautiful zones filled with squirrels make me never want to travel back again. Very few areas are even worth farming due to their localized concentration of worthwhile mobs.

For instance, not including leves there are six map sqares around the Muy-Tun Cellar entrance that I will farm out of the entire zone of the Black Shroud. Obvious example, but I hope it conveys the point. As you mention the zones being full of rats and squirrels are quite pointless.

Roaming aggressive monster packs would be interesting, and your other suggestion to add undead mob spawns during night cycles reminds me of Aion. Both could work well to spice up the world and give reason for travelling to various areas.