While I agree that fixing the "strategic" AI, or even the combat AI, would be a big endeavour, I'm also pretty confident that the "AI builds crap armies" thing *could* be fixed quite easily, with only changes into the army build routines. The AI should weight its decisions mostly towards "heavy" units and discard light ones after a certain (low) number.
This alone will make the solo games *much* more challenging, at least we'll have to fight "real" armies, rather than an half dozen of worthwhile units lost in an ocean of militias ...
