I've always thought that limiting troop type would be good for it - Primary culture = guards, Accepted = regular, Everyone else = irregular.
It's somewhat ahistorical but it would stop countries like UK having huge regular army based on Indian pops.
Except who they can fight will always be an issue, if you are AH and are not dealing with any other rebels, a army full of irregulars would be more than sufficient to put down a rebellion in one of your provinces. The whole reason they formed AH is so that they had two loyal ethnic groups through whom they control control the various other nationalities. But they can easily survive the entire game without taking the decision, and honestly they need to be forced to by circumstances.