One period that I'm interested in, and for which very, very few wargames exist, is the period of 1852-1870. The Second French Empire. Nowhere near as glorious as the First, but the wars from that period are interesting in the same way the American Civil War was - generals accustomed to Napoleonic bayonet charges having to adjust to the vastly superior fire of rifles and the mitrailleuse. The Crimean War, Italian Wars of unification, Austro-Prussian War and Franco-Prussian War might be covered in any potential game in this period.
The only problem is that this period doesn't lend itself very easily to become a strategic game, not as easy as Napoleon I or the ACW, since those were well defined conflicts lasting a number of years. These later wars were short and could not really be fitted into one game. An Austrian victory in 1866 would've meant the end of Bismarck and no 1870. How large should the map be? What to do with the rest of Europe that took no part in these wars? Would Britain and Spain just be on the map and you can't do anything with them?
I suspect any game involving this period would have to be tactical rather than strategic, focusing only on those particular wars. Not sure they would sell very well either...every military buff prefers the first Napoleon to the later one.