I figured I would make this suggestion while Total War, Warhammer 3 is being made. Something that would be a good quality of life improvement is the ability to upgrade a unit of a similar type to that same type. What I mean is, if you have Spearmen in your army, and you want to change them all to Spearmen with Shields, you must remove the unit, create a new unit of Spearmen with Shields, and lose all your xp you may have earned. It is my opinion that in the campaign you should be able to upgrade and downgrade similar unit types into and out of each other, it can cost a little XP and some gold depending on how great of a change it is. In a pinch, if you're in a tight spot, rather than losing your units and XP, you can downgrade them to be more affordable.
I think this is a great quality of life improvement because I would not be forced into deciding what's more important, the unit I want or the XP I have earned. It is hard for me to judge how many units this is possible for across the factions and campaign, however the Empire is for sure the most likely to benefit. I don't know why I always have trouble changing out a unit for a more optimal one, most likely because they have rank 4 and held the line a great deal, but I imagine I am not the only one.
Thank you for your time and consideration, I will continue to enjoy the game, I look forward to 3 very soon!