Just some suggestions for nations combat wise:
USA:
-More divisions, less well trained.
-Mass charge fighting style.
-Mostly infantry and calvary, barrels (tanks) more prominent near the end of the war.
-Aggressive tactics.
-Big honking navy consisting of mostly of battleships and destroyers.
CSA:
-Better trained, fewer in number.
-Defensive tactics.
-Alot of artillery, I can't remember how many times the phrase "fast firing, three inch confederate copies of french artillery pieces" or some variation of that was mentioned in the books.
-Big use of submarines.
-Defensive bonus/fortifications perhaps? Due to the tactics of both nations.
Marxist Rebels:
-Congaree and Black Belt people's republics become small countries
-Chain of events, maybe starting with trouble in the military, then unrest and minor sabotage in industry, then the full blown rebellion.
-Continue to be a pain in the neck even after the republics are crushed.
Mormons:
-Very much the same as the Marxists with a nation color and location swap.
Canada:
-Plays similar to the CSA (although mostly infantry, perhaps even more well trained), although should maybe have an event where they receive British expeditionary units due to the close relationship between both countries.
If you like these, I'm sure I can think of more.
BTW: Sweet idea, I've been wanting to play a Timeline-191 great war game for a while now.