Following the logic. Winter would literally freeze all zombies making them entirely inert. Liquid freezes. Zombies are mostly liquid. The same defrost through such seasons would have the same effect with regard to the undead.
Animals would be mostly the same. Except there would be an increase in dangerous animal activity, not less. Wolves having to hunt more aggressively. Adding more mutants would be nice to liven up the winter months. Balancing out the ranks when zombies are inert.
Stalker creatures are still living. Those would be nice. Imagine instead of zombie hulks. We have Mutant Giants! OH NOES!! Not sure about how the radious tremor attack would work. But a 2 tile tremor attack would be neat and make melee against that beast a real threat.
We also have other planar creatures that come through the portal/tear/breaches. Bees and ants should be even more aggressive. I would be pissed if I had to scoot around naked in winter lol
Dinos too!