I think it should be possible for factions to train/hire friendly factions units (to a certain extend at least). This is what's been done in the fourth age or even in EB https://forums.totalwar.org/vb/showthread.php?t=73928

In warhammer you could have some dwarf/kislevites/bretonians/... units in a empire army (there are dwarf living in imperial cities. Same goes for other factions.

For the special regional based resources it's a good idea. Just keep in mind that there are temples of Morr in every imperial towns. This could be better suited for the Imperial Gunnery School or School of Engineers in Nuln. Such one of a kind buildings/resources/...

This might be better left for release number 2 though.