Logically speaking, dragons are a very cheap unit, as they are assembled from the bones of long-dead dragons. Therefore, I believe that simply reducing the cost of each dragon would be enough to balance them.
As for the Death Knights, I would say that it's pointless to change anything now, but I would be glad to see them in the Necropolis castle in the sixth installment. However, they shouldn't be like the ones we see in the fifth game – the horses should be alive (not with wooden legs), and the lore should be that small horses were trained from childhood not to be afraid of any undead (by the way, this could affect their morale).
Finally, a few words in defense of zombies: they are corpses, so their bellies are bloated from gases, and they themselves are swollen.