Level 8 - :) or :(

Just not like in Heroes of Might and Magic, that would be the ultimate in mediocrity. However, coming up with something different won't be easy, as the seventh-level units in many factions were originally conceived as the strongest, and their history and descriptions fully reflect this. Everything is good in moderation...

I can't disagree with you; Heroes of Might and Magic changed the ancient Greek theme, altering it so that Ghosts appeared in Inferno and Succubi appeared in Necropolis.
As for the eighth level, I would say that making the lower-level units weaker is possible, and you could move the seventh-level unit to the eighth, and place the intended creature on the seventh. The only question is whether the selection of creatures is logical, not absurd.