I've got one.. Had Germany not let the B.E.F. escape at Dunkirk, it is likely UK would have signed a peace treaty; Despite the propaganda in Germany Hitler did not want a war with England. He sued for peace up till the very end.
All the dominoes would have fallen in a quite different manner. For instance, Hitler's war with the USSR would have at least been delayed; it may have not ever have actually transpired (although very unlikely). England's defiance in a strange way forced Germany's hand to go to war with USSR before Germany was prepared to do so. Yet they still came very close to capturing Moscow.
Had the UK capitulated in 1940, even with Pearl Harbor Hitler may not had the reason to declare war on the US; With Germany and UK at "peace" there would be no lingering conflict between the US and Germany. Remember in 1940 and even 1941 popular US public opinion was still to stay out of the war.
One can easily make the argument that Dunkirk was the single most important event in the whole war.
Although improbable due to economics and logistics, there are probable scenarios that the nazi Germany could have won that war.
That one is an interesting scenario. You could make a similar one about the Battle of Britain. Immediately after the fall of France was a rather
dark hour.
Assuming Churchill holds on, I do not see a way forward for Germany, however. The main issue, ironically, comes down to British politics and not anything on the battlefield. How close was Churchill really to going down? I do not think the Germans could have forced the British out of the war by military means. Even if the refugee army at Dunkirk as annihilated, which you correctly note would have been difficult given the Germans were already at their logistical breaking point, then the Germans still need to (a.) chase off the RAF and Royal Navy to open their way to a (b.) extremely hazardous cross-Channel invasion, which would have been hard.
This is why I conflate Dunkirk and the Battle of Britain -- both confrontations are ultimately about if Germany could successful invade Britain. I doubt they could. British air power and sea power was too much, just enough help was coming through Lend-Lease, and running an invasion across the Channel or the North Sea was not something to undertake lightly nor anything the German Army was really prepared to do. It took years of preparation for Overlord; not sure the Germans had a similar patience to be ready for such an ambitious enterprise.
All this is moot if Churchill does not hang on, though. If he is out at PM, then the British might sue for peace, which ends that phase of the war.
Hitler would have still invaded the Soviets at some point. That was the whole point of this thing to him -- his ideology drove him to want the verdant lands to the east, he hated the Bolsheviks, he hated the Slavs, and he personally hated Stalin. It is easier to imagine the Germans winning that war if it was a "one-on-one" between them and the Soviets, but I am not sure if the U.K. suing for peace ends Lend-Lease. The Germans were close to Moscow in 1941, but the Soviet Union was essentially a strategic black hole, with near infinite land, manpower, hunger, and cold for any invading army. Napoleon took Moscow and ultimately lost.
I still think the Soviets grind out a war of attrition on them in the long-run.