This entire post is kind of messy, i am very sorry about it, i hope you understand.
Anything before WW2, no fantasy or American stuff becouse:
-Total War games are about history
-Entire game about american wars would be stupid, as Europeans said during WW1, "they are beginnners". That is true, in Civil war they fought with terrible tactics... Closed formations, they just stand there taking losses instead of bayonet charge- with modern weapons that is terrible. Europeans fought in more open formations, if not more open, at least more wider, and they made more bayonet charges which (before machine guns) saved lives. Also use of artillery was different, but i don`t know much about art. so i don`t say anything about that. American revolution would make booring game, there was absolutely nothing special in that war, so it should be included as part of larger campaign (1700-1850?).
Hm, now that i think, it would actually be interesting to fight "old war with new weapons"... Yes please, but having just 2 similar factions makes things booring, so we should include that too to Empire 2 TW. And WW1, The Great War mod proved it is possible in Total Wars, so yes please.
Side note: Even today, armies fight very differently, French are more aggressive and Finnish more cost-effective etc, so it making game from 1700 to 1918 would give us more different factions that many think.
NOTE: My comment about ACW is not perfectly accurate, and it is not intended to be, as its not the point.
But why the HELL did you not include Empire 2 TW???!!!