How to increase the difficulty of AoE2 beyond the AI

Improving the AI can be challenging, but what if we introduces a way, simple and complex at the same time. My proposal could be a new resource: Iron.

Much like Empires Earth, the Iron can be extracted from "deposits", like gold and stone and requires a mining camp to store it. Of course, you can buy/sell iron at Market and generates at Feitoria (for Portugueses only)

The iron use can focused on military buildings like towers and gates, basically all the techonogies at blacksmith (building when you can store iron), military units from Castle Age (including ships) and techonolgies which improves siege units or buildings like chemistry and murder holes.

What do you think? It's possible to add a new resource to the game in order to increase the difficulty?