My $.02 on an expansion... Ca would be crazy to do anything other than Napoleon as the expansion. It's the only big name-recognition title that can be linked to the game engine without major changes, for example the rifles and longer engagement ranges that would be needed for an American Civil War expansion. The naval battles will still be relevant for a Napoleonic campaign, which they wouldn't be for the Civil War (the South basically had no navy).

An expansion of the current map into Africa or Asia for the sandbox campaign wouldn't be as interesting for most people. It would have no new major power opponents, it's politically touchy (Europeans dominating Africa), and it also gets into that rifled weapons problem. Napoleon will sell better. It could also be more heavily scripted as a canned campaign, which can bypass all the broken strategic/diplomatic AI issues.

I won't buy an expansion unless they fix the strategic/diplomatic AI in the main campaign, but I might consider it if they do.