Something featuring units from the time of the Napoleonic Wars (cannons, muskets, horses), up through the Civil War, World War 1 (primitive tanks, machine guns, those funny, shallow helmets with the wide brims on them), ending around WW II (real tanks, aeroplanes, and a nuclear age superweapon: the atom bomb).
Playable civs should include France, the United States, Germany, Austrio-Hungaria, the Ottoman Empire, Russia, Japan, and Britain.
Rather than minor civs as AO3, perhaps there should be a building more like the consulate or the embassy allowing you to hire men from other countries, including some countries not included in the game list. Ireland and Poland would be prime candidates for "minor" civilizations, while France, Britain, and America would be excellent "major" civ candidates for "mercenaries".
Unless you can think of a way to build trade routes and railroads in game, I would stick with the "trade route" concept found in AO3. But I would start with primitive trains (like the early Norris class engines, or even Stephenson's Rocket), work your way up to more powerful 4-4-0s or ten-wheelers (4-6-0s), and then with the advent of articulated locomotives you can build larger locomotives. Heck, if you're feeling ambitious, you can even throw in primitive electric locomotives!
...[T]he least initial deviation from the truth is multiplied later a thousandfold. -Aristotle, De Caelo
On the lips of the catechist the first proclamation must ring out over and over: “Jesus Christ loves you; he gave his life to save you; and now he is living at your side every day to enlighten, strengthen and free you.” - Pope Francis, The Joy of the Gospel
Call me out if I fail or seem to fail to live up to either of these. Either I will learn something, you will learn something, or we both will.