Around the War of the Ring would be best I feel, as it's the era that most fans are the most comfortable with. If we used a Valhalla style stat/warrior generation system (Making troop skill be any of: Levy, Milita, Trained, Veteran, Hero, Bezerker) then we could make humans one-size-fits-all but restrict specifics based on culture? For example, if you making a warband for Rohan, then your infantry would have to be restricted from "Levy/milita" to "Veterans" to represent that rohan restricts its infantry to either "Villager with a weapon" or "Royal guard" and have the Rohirim be either "Trained" or "Veterans", as they're the main fighting force.