Firstly I'll just say this isn't a faction suggestion and I'm thinking about the future after NW is somewhat overplayed, not right now, it's just an idea for a future expansion.
I'd like to see an expansion pack set in the War of 1812 between the British and Americans.
The War of 1812 was a military conflict fought between the forces of the United States and those of the British Empire. The United States declared war in 1812 for several reasons, including trade restrictions brought about by Britain's ongoing war with France, the impressment of American merchant sailors into the Royal Navy, British support of American Indian tribes against American expansion, outrage over insults to national honour after humiliations on the high seas, and possible American desire to annex Canada. Tied down in Europe until 1814, the British at first used defensive strategy, repelling multiple American invasions of the provinces of Upper and Lower Canada. However, the Americans gained control over Lake Erie in 1813, seized parts of western Ontario, and ended the prospect of an Indian confederacy and an independent Indian state in the Midwest under British sponsorship. In the Southwest, General Andrew Jackson destroyed the military strength of the Creek nation at the Battle of Horseshoe Bend in 1814. With the defeat of Napoleon in 1814 on April 6, the British adopted a more aggressive strategy, sending in three large invasion armies. The British victory at the Battle of Bladensburg in August 1814 allowed them to capture and burn Washington, D.C. American victories in September 1814 and January 1815 repulsed all three British invasions in New York, Baltimore and New Orleans.
http://en.wikipedia.org/wiki/War_of_1812
I think it wouldn't be too difficult to make because NW is set in that time already, and i think it would really please American players. NW seems to be mostly European, I might be wrong but in my observations there seems to be less American interest, but if this was done there might be more interest. I think it should mainly feature a US faction, some different weaponry, maps based on the famous battles of the war, and some different British units too. Possibly Indian units, as they featured quite prominently in the war.
If not an expansion, someone please make this into a mod.