"...Tom and probably others seem to like the ACW..."

To be honest, I don't particularly 'like' the ACW, but keepining it in a seperate forum does mean that when a topic like the Taiping Rebellion or the Indian Mutiny gets mentioned, in the 'Imperial' forum, the posts don't get swamped by threads on whether Hooker was a fool or McLellan had piles.

May I suggest that WAW cover, as it does from 1919 extended, as has been suggested to the late 1960s, to cover the withdrawl from Empire,.
That the Great War forum expand slightly to cover 1900 to the 1920s, to cover the run up to and the immediate aftermath of WW1 (Ireland, Russia, Poland etc.) , this formallises a situation which effectively existas at the moment.
That the ACW forum either stand alone as is or be expanded to cover ALL confict on the American continant (as it was before it became ACW only).
That the Imperial forum cover 1600 - 1902(ish), this will cover from the 30 years war - basically an Imperial attempt to conquer Northern Europe through to the Boer War
And anything else can go into the 'pre-modern'
I feel that a single mark (or set of close marks) such as the American and French Revolutions, 1848 or the Unification wars (either Italy or germany) would narrow the forum too much.

Sorry if I'm waffling (and/or repeating your points)

"The Truth is out there...The lies are in your head..." - Terry Pratchett (Hogfather)