Since Landmarks are a thing, I think it would be cool if the HRE which historically had mercenary armies for a lot of its life, had a landmark that allowed them to make allied unique units for total resource cost in Coin. Just as a gimmick they could do. Since it’s likely that a landmark option will outweigh another.
I know at this point it’s in all likelyhood too late to have any changes like this, I just wanted to mention it.
The mercenary system in AoE III is fantastic. It is just a big sack of content, that can enhance gameplay but can be left completely alone by the player at the same time. It just gives more options, more flavor and identity to the game. It fits the medieval period even more in some way.
And considering lands inside HRE- there’s a potential.