It's probably going to be Napolean, or at the very least CA will expand the map, perfect certain features and include a Napolean campaign. An Opium Wars campaign would be awesome, but it lacks market appeal. TBH, how many people actually know what the Opium Wars were? On the other hand, everyone knows Napolean and everyone knows the American Civil War. But still, the combat in the American Civil War is drastically different from the 18th century, and I don't think the Civil War will get the expansion (the ACW will get something, either now or later, though).

But yeah, I think it'll be Napolean. The Napoleanic Wars are more spread out, you can have campaigns in Turkey (Anglo-Russo-Turkish wars), Russia (Napolean's invasion of Russia) and even Haiti during their revolution, that way they don't have to scrap the whole map and start anew.

Personally, I'd enjoy an Eastern European campaign (the Serbs, Ottomans, etc.), a Zulu Kingdom campaign (not only the war with the British though), and a campaign about the South American Wars of Independence.