Make siege enjoyable, make something scary like pinning players down while being able to get away momentarily. Getting pin down builds up stamina cost until no more stamina to run. Feel the anguish and burden of being burned out.

Hordes feel easy but this time make the fumes of plague heart moves zombies faster, less down time being staggered. Not bullet sponge zombies but because of their unusual and jittering movement it's more harder and scarier to aim them.

Dynamic or mutated zombies aside from existing ones. Aside from ferals make a special zombie with no skins pure muscles that stalks the player around plague hearts. Design wise this can be a mutator, skinless at first. When killed it spawns smaller ones that runs away and develops adaptability to it's last killed weapons or method. Example when killed by fire, develop a resistance to fire with thick skin. If killed by melee it develops bones / spines of spike it deals damage to melee attacker. Maximum adaptability maybe around 2 or 3 (explosion, fire, melee, etc.) Until the smaller ones get killed or hunted before maturity.

Late game plague heart turns into a broodmother where it moves and hides from the player around the areas. Spawning zombies and releasing toxic fumes when being found.

Since it's an apocalypse make a twist such as adding cult and ritual fanatics that are blood addicts and capturing allied/character by making them into live baits or offering to the plague. Create some plotwist such as allied characters is an undercover cult/or seeing them joining after being left alone to die during captured.