The Nazis might well have tried to conquer more and more land, but to actually DO so would have required a strong and growing economy. Wars are won by the ability to pay for the toys that make them possible. Hitler payed for his Third Reich via credit, and by 1939 the bills were due. It was fight, or go broke. To the man, his generals wanted to wait a couple years before launching the war, knowing full well Germany wasn't ready. What they didn't know, but Hitler did, was that the coffers were empty.
Germany could barely push into Russia, let alone win it. It never could have held the country.
The US got involved in order to prevent most of Europe from falling to a totalitarian Germany. Instead, after the war, it fell to a totalitarian Soviet Union. The end result was pretty much the same. And, the long-term result of a nation under state-planning would have been the same, as well.
In any case, as with all WWII "what if" scenarios, all things fall apart under the fact that by 1945 the US had the atomic bomb--something Germany abandoned early on (and was barking up the wrong tree anyway).