Question: What was Nazi Germany's end goal?
Short Answer:
Hitler had a long history of advocating for multiple wars and stated so in both his books. His best selling Mein Kampf and his uncirculated unreleased "The Zweites Buch". In both he discusses step by step plans for escalating wars ultimately becoming global war and genocide. Hitler's endgame changed with respect to the United States between books, and was altered by reality with regards to the UK. Hitler was consistent that Germany would become a larger player on the world stage through war at the expense of eastern Europe, the Soviet Union, and France. He was also consistent in both book with regards to Genocide against what he called inferior races.
Detailed Answer:
Hitler wrote 2 books ("Mein Kampf" and "The Zweites Buch") and in each he discussed the need for Germany to pursue multiple wars. In both books he outlines what later became known as Stufenplan ("stage by stage plan") for conquest and genocide in Eastern Europe, and then the world.
In the First "Mein Kampf", Hitler expresses his "passionate belief" since youth, that Germany must obtain a larger place in the sun with the help of the sword wielded so efficiently by Prussian kings. Hitler responds to people who claim that a second European great war would mean the end of Europe, saying it was only "eternal peace' which destroyed peoples and that "neither the individual nor society could escape Nature's decree that the fittest alone survive".
Three major themes of "Mein Kampf" or My Struggle all deal with the need for War.
(1) To avenge Germany against France for the harsh treatment after WWI through renewed military conflict.
(2) Germany's need for "living space" and the military expansion into the eastern slavic countries.
(3) The need for a bloody crack down on jews and other groups he considers inferior.
Now alternatively Hitler does famously allude to his plans for the UK and the US.
He calls the UK a "stabilizing influence on the world" and basically a net positive so no need to destroy them. He alludes to the UK as a natural ally for Germany against France. He calls the US a "country of Mongrels", no threat to Germany and thus, no need to destroy the "American Union".
Hitler Second Book, "The Zweites Buch",
War was definitely on Hitler's mind..
He downplays the costs of wars and paints them as inevitable, desired and necessary, as long as they aren't "eternal".
The Zweites Buch
"Indeed, the losses which arise directly from a war are in no way proportionate to the losses deriving from a Folk's bad and unhealthy life as such. Silent hunger and evil vices in ten years kill more people than war could finish off in a thousand years."
and
"Therefore, wise political leaders of a Folk will never see in war the aim of the life of a Folk, but only a means for the preservation of this life."
In this book he doubles down on the UK as a natural ally and writes them in along with Italy as his allies against France and the Soviet Union. Also in this book he changes his position on the United States. In this book he observes that a million Germans immigrated to the United States. That they were among the best Germans, the risk takers. Those who gave up everything and risked leaving the Fatherland. So he reclassifies the U.S. as an existential threat to Germany's future, which will need to be taken out. As in Mein Kampf, The Soviet Union remains Germany's greatest near term threat, here Hitler claims the US is now Germany's greatest long term threat.
Zweites Buch :The "Fourth Stage"
In contrast to Mein Kampf, in Zweites Buch Hitler added a fourth stage to the Stufenplan. He insinuated that in the far future a struggle for world domination might take place between the United States and a European alliance comprising a new association of nations, consisting of individual states with high national value. Zweites Buch also offers a different perspective on the U.S. than that outlined in Mein Kampf. In the latter, Hitler declared that Germany's most dangerous opponent on the international scene was the Soviet Union; in Zweites Buch, Hitler declared that for immediate purposes, the Soviet Union was still the most dangerous opponent, but that in the long-term, the most dangerous potential opponent was the United States.
Hitler's endgame changed, but was consistent that Germany would become a larger player on the world stage through multiple wars escalating to global conflict. His principle targets for expansion would be, but not limited too, eastern Europe, the Soviet Union, and France.