Well Imho one has to differenciate here between Equipment, Population and Tradition as units often(almost exclusively) represent a fixed setting of the three.

A good example here is the Hoplites: A guy in hellenic equipment, of hellenic orgin fighting in the traditional Hoplitai way. If you interchange one of these you get a different unit - Saka HH, Kardaka(pl?), Thureophoros (in EB terms)
As the Equipment is not really hard to get (not made from special material or smithed in a funny way) and Tradition often travels along the people, you can train Hoplitai In regions that had a significant Hellenic population - thus Hellas, Successor states and old colonies all around the mediterranean(and Black sea). To train Hoplitai in .... Scotland. You'd have to migrate a large chunk of hellens to Caledonia let them settle(and breed) there for some time and finaly build a level 3 MIC^^. It's hard to simulate such a Migration in TW games yet I think It'd be cool to make a building(line) availible that unlocks recruitment of units like Hoplitai everywhere a Hellenic Power settles for a Generation or the like.
For population this process is rather "simple"(if you consider relocating whole peoples simple), Tradition and Equipment are a bit more tricky, Either they go without saying or they simply don't, at all.
Tradition for example, you can teach every populance how to fight in a phalanx, some may be better at it some may suck at it but basically it's not that big a problem, turning a bunch of shepard into skilled archers takes far too long for any politician to grasp, infact it takes about the lifespan of one and thus is not an option for TW.
Equipment fares similar, if provided with the tools your civilisation brings with it and maybe 10 imported specialists you can just aswell make Aspis Dory and linothorax in Gaul or anywhere else they have the basic raw materials or suitable replacements. But if you need something more unique, like ... Yew bows, Meteor iron swords or simply put Elephants you are reliant on Imports. So you may field a unit of ... Gaut Elephant Bamboo longbowmen with Silk tunics in Sweden but It'd cost you a heck lot of extra money to get all the stuff, IF you get it at all. In some cases it may actually be possible to build up certain industries that did not exist in an area before.

In the End The EB policy "no fantasy units" is a main restricting factor. And every unit that did not exist in a certain region IS a fantasy unit, the Saka hoplites eg did not exist on the Peleponnes, for the simple reason that the sakans did not conquer the peleponnes. IF they did they might have used units like that, they might have used Hoplitai of this Type but they might aswell not for various reasons. In a way a unit is a different unit when it was trained in a different region, so scottish Hoplitai ARE fantasy units. I think the EB team wants to be on the safe side without loosing all the fun that reforms and expanded AoRs bring. That's why they only added units that actually existed(at least as far as I know) or are a safe bet. Personally I think that there should be more optional reforms with expanded AoR or enhanced units - It's not like the Germanics would have sticked to guys in shirts with spears when they'd have conquered the area of Charlemagne's empire and relocated themselves so that you can train Factional troops in northern Italy. But as this did not happen and is open to specualtion on how it would have worked you can only train Germanic troops in, well germania.
Secretly I hope that through more room for scripts the EB team will add some enhanced units and expanded AoR reforms to EB2 tho^^