Because the Border Princes are an apparent Mad Max wonderland with a mish mash of a whole bunch of faction's outcasts and mercenaries I think the Border Princes should be able to actively recruit Empire and Bretonnian units from their settlements as well as the appropriate Rusichi units and maybe even a few other faction units like maybe a few Dwarf units.
This shouldn't be too hard as all you got to do is add those units to the rosters and such, right?
Also I think that the Border Princes should be granted a higher chance of getting those scripted Hire events of the Dogs of War because those Border Princes will want, need, and have the cash to hire mercenary units in bulk and so that would almost be the first stop for crack units of swords for hire.
These are my personal suggestions! I hope you take them under advisement! Keep up the great work guys! Amazing mod so far!