The Lord Of The Rings: The Rings Of Power - 11 Things We Expect To See
5. Flat Earth Becoming Round
As strange as it is to say, the world in which Tolkien's stories took place, Arda, actually used to be flat. And even after it became round, it was still technically flat for the Elves, which gave them the ability to see long distances and navigate to their homeland / heaven at the end of their time in Middle Earth.
This event is most likely going to be shown since it's part of Numenorian history. Maxim Baldry has been confirmed as Isildur (best known for his role NOT destroying the One Ring at the beginning of the films), and a young Isildur was present right before Numenor's destruction.
The reason the flat earth is made round for everyone except Elves is because the long-lived Numenorians, the race of Men Aragorn descends from, were tricked by Sauron into attacking Valinor (Elf heaven and the home of the Valar, their gods) to seize true immortality.
In response, Eru Ilúvatar (their version of God and creator of the universe) drowned the island of Numenor and made mortals unable to travel to Valinor. Only a few Faithful survived in nine ships, and Isildur was among that number.