But then it wouldn't be Diplomacy.
Look at any strategy game. They can all be pretty much reduced to Risk or Diplomacy. You use [military forces] to capture [provinces]. Whether it's infantry, cavalry, or just armies capturing provinces, territories, zones, of whatever by supporting moves, having stronger invading armies or by chance, the're all variants on the basic formula.
If you did have different unit types, would one be stronger than another? If so, bang goes the skill of supporting moves and holds. If one moves faster or something, it throws everything out.
So- yes, you could design a game with stuff like you suggest, but it would need changing so much it wouldn't be Diplomacy.
But I'd play it for sure. :D