Perhaps this kind of thread is now a bit futile, as I suppose CA have probably already considered most of what the expansion will bring (and they'll have us believe it's too time consuming and expensive to make other changes), but here's some of my suggestions:

I think what we would all basically like is more control over the game: from the bonuses and penalties that can be applied to buildings, provences and units, to the effects of fatigue, weather on weapons on the battlefield etc. We should be able to do more, and also have a lot more control over events without needing scripts running in the background. The list could go on.

I also think the modding community would be very grateful if CA would enable us to use much more of the Romeshell commands, particularly those that enable us to switch factions, autoplay the game and so forth. And also let us have the Campaign/Strategy Map editor. We know there is one - there are graphics for it and text already. That would make modding so much easier.

There's a load more things CA could do for the modders, but most of them I can think of at the moment basically boils down to the same thing: open up the game and let us have more control instead of there being so many hardcoded features.