Yes, having certain units with stealth could be cool. If an entire army is composed of stealth units, it could be hidden on the campaign map ... and any stealth units would be hidden on the battle map.
The way I envision "hidden" to work, is that if you know where to cast a reveal spell, you'll be able to spot them, but otherwise your units can only see them if they are right next to those units. So theoretically some assasin units could "try" and sneak past ur army during a battle to attack your sovereign, but once they are in a tile that borders the enemy (near the enemy) they are visible ... but only for as long as they border an enemy tile.
Having a Champion (or Sov) with stealth would be just epic .... 
