Honestly, Civil War is pretty much unknown outside of the US. I'm not sure CA would release an civil war-only based expansion.

Furthermore, it would only allow 2 main factions, with possibly a few more (France, Britain, Mexico, though neither of them had a really big impact). If an expansion taking place in the Americas was to be released, it would have to not only focuse on the Civil-War, but also on the Republic of Texas, on Mexico, and on the independence of Southern American States (Bolivar is obviously an interesting character who deserves to appear).

On the other hand, the Napoleonic Wars offer an interesting setting, pretty much known by everyone. And though most of the fighting took place in Europe, the US fought in the 1812 war.
Moreover, a Napoleonic era would probably require a lot of cool new options, like setting your brother as monarch of another country, or offering 'national ideas' a la EUIII.

I could settle with any of these (though I'd prefer a Napoleonic era one, as a frenchman), but I'd rather not have a Kingdoms-like expansion. At the end of the day, most new campaigns were watered down, with only a few factions, and were far frow challenging.