What if the US was able to push through a plan for a cross channel invasion in mid 1943? Let's say that they work out a plan with the British to put 3 US, 2 British and 1 Canadian division across along with 3 airborne divisions in late July 1943. Now, this doesn't have to be Normandy per se. They could invade along the coast from say about Calais to Bordeaux.
Of course, there would be no invasion of Sicily although the Allies might keep such a possibility as a deception to draw off German forces. Certainly, the Allies had the manpower to do this. They likely could have scrapped up the shipping too, even if it meant a slowdown on operations in the Pacific.
The Germans would also have been in poorer shape to repel an invasion. More of their armored forces were in Russia engaged in the Kursk operation. The Atlantic Wall was a mere fraction of the strength it would be a year later.
So, the question is could the Allies have pulled this off...eg., get ashore and stay ashore? I think they could have. I also think that they could have pushed across France by early 1944 and been on the Rhine nearly a year earlier than they were originally.