I love the AOR system it adds so much depth, but I do have one suggestion
To put this in to context, I'm currently in a Seleucid campaign and I have taken over all of Asia Minor. Rome have taken all of Hellas, Macedonia and Thrace, so I feel like we may be at war soon. History tells us that Roman legions really struggled against the horse archer heavy armies of Parthia. History also tells us that the Seleucid's often used horse archers from Dahe.
To recruit these horse archers to fight against Rome I would have to march a general from my recruitment province in Syria, all the way across to the Parthia area, and then all the way back and up in to Asia Minor, and that would just take way too long.
There should be a way that you can recruit all AOR units in one place. Obviously this has to be limited otherwise it defeats the whole point of AOR, and thats not at all what im asking for as I love the system . But perhaps there could be an edict which you can use in only one province (perhaps your capital) that allows you to recruit all your AOR units while it is active, therefore the balance is it comes at the cost of another edict and can only be used in one area. Another way could be a high level research technology, or a feature of level 4 city centre buildings (again capital only) therefore it would only be available in the very late came