Wasn't this done in STW? I'm not 100% sure, but I seem to recall that certain units needed more than one type of building to make. For instance, heavy cavalry needed not just a stable but also an armoury, which could only be built in certain provinces. I think the same went for Naginata. Training troops didn't deplete "pools". However, the system of limiting provinces which could build armouries and what have you meant that not every province could field elite units.

I might be totally wrong though...