I think the next DLC must be a "Art of Peace", with some mechanics for peacetimes.
To be fair, a large proportion of the additions in the DLCs (including Art of War) have been peacetime mechanics (although unfortunately, many of them have been focused on 'special' mechanics for specific countries, like Dutch Republic, rather than improving the base experience for a 'standard' country). But these are always going to be tweaks to what is a very limited setup, especially at the local level. I don't see how we will get a sophisticated peacetime game without a complete overhaul of how provinces work, by bringing in the POP system from Victoria 2, and/or by making the building system much more sophisticated, with more impactful choices (developing your provinces shouldn't be an optional extra like it is now).
One way I'd like EU4 to be more like Civ is to have a situation where provinces are almost always constructing new buildings (assuming you don't have a pressing need for armies/ships), but each building takes a long time to complete, and the rate of progress is affected by various factors (e.g. autonomy, stability, admin efficiency and so on). Also, get rid of 'lines' of buildings, just have lots of different buildings that all do different things. That way, you have to think about build order and priorities. You could make it so that buildings don't cost monarch power in the first instance, but you can pay MP to hurry the construction.