I know this is a touchy subject, and will probably be rejected right off the bat but... I believe other major cities within a realm should be able to lend themselves to recruitment. Of course the capital would be the best place to recruit, but I feel it is an odd restriction and breaks away from the flavor of the game.
There are several ways to balance this, one being increase the cost of recruitment, make the average quality of troops lower, highly restrict the number of men that can be recruited this way, and perhaps because these units would have mostly green individuals make it harder for them to train/their equipment would break faster.
Possible exploits: Would disrupt the pendulum of war and give an advantage to large realms.
