A late 19th to early 20th century/Victorian era/age of imperialism game with most European countries and many African and Asian tribes and countries as factions would be really cool. It wouldnt be completely historically accurate since you would be able to have countries that never fought against each other at the time fight agaisbt each other, but it would have accurate uniforms,troops, and units. It wkuld also feature artillery, mounted combat, and iron sights on rifles that would range from repeater rifles to beech loading to early bolt action rifles. The combat could be based on several different wars from the time such as the Boer war, boxer rebellion, and some other colonial wars that took place at the time.
There really aren't any shooting games set in the Victorian era so a game like this would be really great. On the other hand, I am sure an idea like this could be put into a mod for BCoF since it would be basically the same gameplay and game modes, having large multiplayer battles with different types of units, artillery, and basically just big battles between two factions like nw does.