[Taqwus]

I'd agree with Wick here -- this is the sort of thing already abstracted by province defense.

That's not to say that province defense couldn't use changes, however. For instance...

...should province defense correspond to national troop types, or to the locals, or a mix? This gets particularly odd with the underwater races.

...should the maximum be based on the size of the province, or its population, et al? Or, should the maximums remain the same, but perhaps increasing the cost and strength?

...should it cost maintenance or supply?

...should there be a per-turn limit on how much it can be increased in a single province? Should unrest play a part?