I agree, Napoleonic seems the most likely first expansion...but I hope CA doesnt let that cloud them to other possibilities. There are a lot of civil war buffs out there who would love it.

You are right, the mission based campaign would seem a logical way to go. But I can also see it working in the style of a grand campaign, with just a more limited map (divided into territories by state), and well defined victory conditions (North-capture richmond and/or force surrender of major southern armies, South-force political solution either by holding out long enough or by capturing washington dc). The north would have an advantage in naval force, industry, and trade. The south would have better unit morale, better generals, and advantage of being on defensive (choosing terrain on which to fight, etc.).

In many ways, it would be a "cleaner" game. Many elements currently in the campaign would play no role (no point in fitting in diplomacy, religion, etc.).