Some thing, that I have found to be quite annoying was, that I could not select which specific unit I want to put in the army I create.
For example I wanted to take the spare horse-cart with me, so I could add in more gold for trading. Turns out though, that my army took the one, that I had positioned near the goldmine instead of the one I had just bought (the one that was standing around with no purpose).
Easy way to circumvent this issue:
Allow players to add all the units they have grouped together via hotkey at once without having to select them. To explain this:
I would have grouped my horse cart with my army under hotkey 1 and would have added the entire group 1 into my army. That way I can specifically select what unit I want to be gone and which unit I want to keep at the specific location it's standing at.
I hope I wrote it in an understandable way