I think, a War of 1812 mod would be nice. British/Canadians/Natives vs. Americans/Natives/French. "1812"
Or, a Roman Mod based around the first Punic War. Roman Empire (Was not a republic yet, I don't think) vs. The Carthaginian Empire. "The 1st Punic War"
Or, a WWII mod, based on the Eastern Front in Europe. Germans vs. Russians. "WWII: Eastern Front"
Maybe even, a medieval mod? Based solely in the UK with 5 factions... Kingdom of England, Kingdom of Wales, Danish Raiders, Kingdom of Scottland, and Kingdom of Ireland. "Supremacy"