Modding extra challenge into the campaign - what's the best way to do it?

[Glabro]

With all the campaigns completed on Hard with little to no reloading, I'm looking to revisit the game in the near future. The map editor can edit the campaign missions too, so I've been wondering on how to improve the performance of the AI and generally make the missions more challenging, preferably without simply making all enemy units harder (but if that's the only way, I'd like to figure a pattern on how to do it).

Could the AI be made more aggressive, or can the fog of war be eliminated so the AI knows where to fight you (as do you), what else could I do? I'd like to hear your thoughts, map makers.