There is one very simple change they could make to the AI that might make things better. I saw it as an option in computer Risk (must have been years ago), and it was simply that if you took a territory off an AI player it remembered that was "its" land and basically threw the kitchen sink at getting it back. (And for MTW also code it so that there is none of this "The XXXX decide they cannot win the battle" nonsense.)

It doesn't sound like much but believe me you could beat the AI with your eyes shut with that option turned off, but with it turned on it was almost impossible to win.

I think its right that the AI's economy is half the problem, to be honest although its a total cheat I wouldn't mind a crude system that just gives the AI the average of what a human player would get from a given province in a given year regardless of what it does with its trade ships or whatever. Also a complete ban on the AI building peasants would be no bad thing.