Yeah a mod will do it, I don't think TW will. It would be so so so cool if at the very least when the player takes fiefs they could combine their cultures, or do something to import a notable or such so that you could have recruits form different troop tress or unlock a special troop tree. Pie in the sky.
Who else is sick of all the ****ing imperial recruits!? Doesn't matter what faction you joined chances are the recruits from your fiefs are gonna be imperial recruits!