I have an idea for the Fallout RTS. Maybe you could start before the Great War in one scenario, but after the war in another, possibly a DLC? The Company of Heroes style would be easiest to make I think since you could focus on making small, action packed maps.
Here are a couple of scenarios:
The Resource War/European-Middle Eastern War
Anchorage Campaign
Canada Annexation
The Great War
Battle of Hoover Dam
NCR-BoS War
NCR-Legion War
Lyons BoS-Enclave-Mutant War
Another slightly better idea would be closer to Civilization 5 or Crusader Kings 2. You would either guide a family throughout the entire Fallout timeline, or simply manage the world itself. The family idea would be focused on keeping your bloodline going until a certain time period, while the world idea would be focusing on getting humanity through the Great War and rebuilding afterwards. I imagine building various settlements throughout the wasteland, while defending from various ghouls, mutants, raiders, along with the various other factions vying for supremacy as well. No faction would be left out either. It would follow all recorded Fallout timeline events, from the trivial like the Great Passion Fruit Famine of 2044, to the major ones like the Second Battle of Hoover Dam.
* I just had a neat idea. I wonder if someone could work up a Fallout mod for Crusader Kings 2? Maybe just focus it on the US though? Hmmm...