Half of this stuff could I think be best dealt with by increased game options in the lobby.
But the mentioned idea here sounds like a good one-- being able to load up custom rule sets from the resoure centre. If this included the option of extra units it could make a big difference to the interest in them I suspect. I mean right now they exist but it's particular maps... To be able to play any map and load up custom faction types with lots of extra units could just explode that side of the game.
If the devs implement mutators, what kind would you want?
Brainstorming session.