I was thinking about this as well. You should be able to make basically any unit of the game without making them OP like battanian fian champs are
It would be so awesome to be able to create the army you want for yourself, BUT the only point is how do you balance them for recruitment? because now you only get units from their respective cultures from that fief, so battanians from those from battania etc if you start your own kingdom, do you convert those towns into your culture and can you only recruit your own then from those fiefs?
I would say normal recruits should give max tier 5 units with a noble branch that gives tier 6 for a bit more oomph. Those should be just a tad less statswise then the current ones are so no copy paste fians or cataphracts.