ORIGINAL: rhinobones
Push the invasion date up to 1939-1940 before the US rearmed and Germany became involved in France and Russia. Think this would tend to make the assumptions a bit more palatable.
This sort of assumes that Britain and France just wave the German invasion fleet past- and that Russia happily sits on her hands while Europe invades America.
I don't think it works. Frankly I don't think any timeline works for Fall Grau as it has all sorts of impossible things happening. If you wanted to make a remotely realistic scenario then you have to assume that Germany simply
crushes all her European opponents by the end of 1941, which isn't all that far fetched and many at the time expected exactly this to happen. Then one has 18 months of build up where Germany for whatever reason decides to go off and
crush the United States as well, establishing various bases in the Caribbean which can then be used to advance up through Cuba and Florida, and possibly via Mexico. Any other landings would be totally unsupportable due to the need to get land-based fighter cover over the invasion area, so this gives us a very different scenario where the Axis player has to more or less follow the script and fight his way from port to port and airbase to airbase, trying to get out of the Florida bottleneck before the US can mobilise.
For now,
Fall Grau sits broadly in the same category as your
War of the Ring scenario: something that's entertaining even if it's not exactly simulation. Actually, if you assume Hitler has found the Ring of Power...