Due to the increasing abusing in mixing different races best troops (example: dragon turtle is way too powerful combined with rocketeers) I would like to suggest to limit borrowing ONLY within the same race.
If you change race spending honor, borrow will be automatically revoked.
Many people are starting another or multiple accounts just to permanently borrow dragon turtle worth 35 Gold troops cap that will cost you much if you want one within your gold troops cap (or you should just choose the rakan race) and this is very unfair for someone who just play one account, as the game is meant to be played.
Another idea is to add a BIG handicap if you decide to mix 2 or more races, something like -50% attack speed and movements, or starting battle with bad morale (but this won't affect mechs or undead, so better the first idea)
I think this change will balance the game a lot, and devs will be able to balance races without thinking that someone can just borrow the best troops from races to be unstoppable and easy to win.
Feel free to add more ideas.