I'd like the following things in an expansion:
1)Diplomatic option to purchase provinces. Purchasing territory was done quite often in history and having the option to sell without the option to purchase(and the AI never offers) is annoying.
2)Balanced peace deals. Instead of white peace how about peace deals where you can both demand and offer tribute. So in a peace deal you could give up some of the land your opponent took in exchange for some of the land you took, or maybe add some ducats to the deal so your war score is enough to claim a strategically important province you would need to war for years more to attain.
3)I'm also a fan of the dynasties. Having an outline of your family and being able to choose successors would be awesome.
4)The ability to rename your country without changing your tag. This would likely be huge for mods, as from what I've heard, having a lot of tags can really slow down a mod, it would also be nice for people who want to use a made up country without going through the trouble of modding it in. Its already in for provinces and capitals, why not countries?
5)The tag console command from Rome. I cheat, and this makes it a lot easier than saving, resigning, loading new country, accepting bloated peace deal, saving, resigning loading old country.
6)No more terra incognita. Use other methods to disuade colonization etc. such as high movement costs, attrition, etc. rather than big ugly grey blotches.
7)I want to be able to choose which provinces a vassal gets when I release them.