Veteran creatures.

Nomads and spearmen were present in all parts except for the 5th. Elementals were in all except the 1st. Dragonlings are a classic; how could we do without them? They just changed in all parts, except for Shades of Darkness, where they remained the same since the 2nd.
They forgot about rogues, and that's unfair; they were always there (let's assume that in the 5th, they were replaced by assassins). In 3 out of 5 parts, there were druids, medusas, hobbits, ruhks, and death knights.