I didn't know what to search for, even though I'm sure I've seen something relating to this.
But is their a mod that only allows certain units to be built in certain places, but when you take over those areas you can make similar units.
Such as, Romans only being able to recruit legionnaires in Rome (or around Italy), and only being able to recruit auxilia else where, as well as local more barbarian units.
And say, if another "civilized" faction took over Rome, they'd be able to build some simple version of roman units (of course not the special units).
Of course, a Barbarian faction taking over Rome, the whole place would likely go to , so they probably wouldn't gain the "tech"/training to do that.