I think that we should have both resistances AND Immunities.
Resistances should grant +4 arm vs fire/cold/corrosion damage rolls(and change corrosion to be a D3 damage roll, with resistance making it D3-2, so you only take 1 damage 1/3 of the time)
Lots of things which currently have immunities should have them switched to resistances.
Protectorate Reckoner Chassis: Resistant to fire
Feora and Malakus: immune to fire, resistant to cold
Cleansers: resistant to fire
Azrael: resistant to fire
Pyre/Slag/Winter Trolls: resistant to fire/corrosion/cold
Fire Eaters: resistant to fire