Which units would you like to replace/add?

I suggest removing the dragons again and replacing them with Death Harbingers. And replacing the Death Harbingers with Death Knights with their old abilities. Give the Death Knights 130 HP and the "Death Touch" ability. Increase the Death Harbingers' HP to 150 and give them the "Double Death" ability: the first attack with Death Touch will kill 2 units at once. Give the Archliches a 10% chance to cast a random curse (or only plague) when attacking with a gas cloud.