I started work last night on a mod using the hidden resources to limit where factions can build their troops. Personally it just doesn't feel right to me for Egyptians, for example, to somehow end up conquering Gaul and start being able to locally recruit Nubian Spearmen and Camel Archers. Yes, over time the conquered people might be replaced or adapt to their conquerors ways, but sometimes the reverse was true with the conquerors eventually becoming nigh-on indistinguishable from those they once vanquished. So there are perhaps arguments for and against my mod.

The individual province hidden resources are easy, although I'm sometimes flipping a coin for the cultural/regional groupings since I'm not doing any major research at this stage, just setting up the basic mechanics. One further idea I'm possibly going to add or use instead is another level of hidden resources that take account of the typical terrain of a region... it seems a little odd to me to recruit steppe-style horse archers in the mountainous areas of europe, or camel units from the freezing wet north.

My current idea is to make the core, elite units of each faction only buildable in their starting provinces plus selected neigboring or similar regions, eg. horse archers would be recruitable in any of the steppe regions. Lesser units, the Roman's Auxilia being a perfect case, would be recruitable anywhere.

I do have one major worry and that's how it will effect the composition of AI armies. Since they can only recruit their best troops in limited areas, they might end up with the equivilant of MTW's peasant horde, but for differing reasons. In addition I doubt the AI will be clever enough to know it has to send its units back to the home regions to be able to replenish battle losses, further weakening them.

So far I'm just putting it together, haven't seen how it plays yet, so there'll no doubt need to be lots of tweaking.

I also intend to address a few other issues in my mod: ditching the fantasy units, amending the oddities such as the armour of the desert axemen, and making naval combat more decisive, for example.

A possible future extension - although one that would require me to learn how to edit textures & sprites - would be to produce versions of each faction's troops, probably with reduced stats or higher expense, that conquerors could recruit, although this is brushing too close to fantasy for me to be completely comfortable with the idea, plus I might find the AI so adversely effected that I have to drop the idea anyway or limit the effect to special cases, such as camels only in deserts, etc.