Yeah I think a regional recruitment system would work well for Auxiliary troops. I think that some regions should give you the option to recruit a unit type regardless of what faction you are playing. So no matter who you conquer Greece with, you should be able to recruit Hoplite Militia from Greek cities and so forth. And if you recruit a legionnaire unit in a Germanic province that you have conquered, it should say 'Germanic Legionnaire' and they should get some bonuses available to Germanic units such as 'bonus fighting in winter' and so on. I think this will add more depth and force you to think about where you recruit and how you recruit.