Re: Suggestions about recruiting units
It is called AoR (area of recruitment) system. Normally based on the use of hidden resources or trading resources as condition for recruitment. This system has been used in that you can only recruit imperial troops in the initial capital, elephants only only in regions that have that trade resource etc.
There isn't a huge culture diversity like Celts, Greeks, Egyptians were your suggestion would be perfectly viable (and easily noticeable), but we have made use of the principle.