The trick, then, is that ‘surrounded’ aspect, right? Maybe Master Zombies and such could project better AI at their minions? The thought that comes to mind is something like the Brain from Deadly Rooms of Death… which though otherwise fairly harmless, turns mindless and easily-defeated enemies into a deadly threat. Consider the idea of a Master Zombie, with senses better than your own, sneaking around beyond your vision, rounding up mobs of lesser zombies, following your scent trail, tracking you back to your safehouse, maybe even waiting for you to sleep… and then suddenly you’re awakened by a siege in the darkness, and have to fight or run… and if it looks like you’re winning, the Master is already in retreat, looking for another horde to gather.
Having zombies that are actually able to track you by scent like that, perhaps like in Rogue Survivor, would do a lot to keep them threatening, since it’d be harder to run away (they keep coming, and don’t stop to rest), and since they’d be able to attack you while you’re vulnerable.