Hmmm.... wouldn't actually make that much more work. Just mean that units are region-specific and are listed as being buildable by (almost) all factions. You'd want to leave some of the more specialist units to faction-only. Gauls should not be able to raise Urban Cohorts, no matter how hard they try. ;)

Also, it might be good to limit the cross-over to highly diseperate cultures. Romans able to recruit horse archers in Parthia, but it would only take sufficient indoctrination and incentives to get Britons to serve in a Legionairy Cohort, as they did in history.

Of course, much of this is already allowed through mercenaries. To mirror this, you could simply disallow any faction all but their most basic units in all but same-culture areas and make mercenaries more diverse, cheaper and more common.