I voted Dunkirk 2 with the stipulation that Rommel had a free hand during the landings. I think he could have pushed us back into the sea. Now, the question is would it be a dunkirk 2? Only if we were lucky. Merely stopping an invasion was not the sole goal of Germany, more not allowing another dunkirk to happen. IF (and only if) the bulk of the landing forces was either taken prisoner or destroyed there is a chance it could have altered the outcome of the war. I don’t believe that Britian and the US would have made a seperate peace, but I do think that for quite a long time there would be virtualy no threat on the western front. Thus, troops get transfered to the eastern front. The new influx of troops may have stabalized the front. I think this would only have been possible if Generals like Rommel, Kesserling, Manstien, and Guadarian had a free hand instead of having to answer to corpral Hitler.
Even a longer war could have had drastic changes in the outcome. Imagine fleets of ME262s stopping the air war over Germany while Britian gets battered senseless from V-2 rockets. The type 9 subs that were equipped with snorkels were a harrasment to the allies. Imagine if they would have come out in force? Would have been like 1940 all over again. Not to mention the Germans were very close to having surface to air missles. And, it can be speculated that the manhattan project would not have recieved as much financing if the allies lost huge on D-day (resources would have been needed to replace losses).