Hypothetically: which units from Heroes of Might and Magic II would you replace?

Well, in Heroes of Might and Magic III, they simply buffed all the other starting factions, and also nerfed skeletons. However, after that, Necropolis became almost an overpowered faction, while in Heroes of Might and Magic II, with such a powerful starting army and the potential for stronger armies later on, I wouldn't say that Necropolis was even in the top three factions.
And it all stems from the fact that they wanted to balance the factions and make the heroes more diverse. As a result, they ended up with factions that have varying strengths and a lot of overpowered heroes. So, not all good intentions lead to the desired outcome.