Uatu was initially conceived as being a cold, stoic observer of the show's alternate realities. After Jeffrey Wright described the experience of watching Marvel movies with his kid however, the character was re-written to have a warmer and more devoted interest in what he was watching instead.
An episode pitch involving the Guardians of the Galaxy was suggested for the first season, but it had to be scrapped after A.C. Bradley was told that she had inadvertently written half the plot of Guardians of the Galaxy Vol. 3 (2023).
One episode idea that A.C. Bradley came up with would've involved Loki becoming worthy to wield Mjolnir. This idea was discarded early on however, as the idea of Loki becoming a hero was already in the works for his own show, Loki (2021).
There was going to be an episode that would've consisted of Spider-Man turning into an actual spider, but the idea was scrapped as A.C. Bradley found the idea to be far too dark in comparison to the rest of the series. The above event also has already occurred in the reality portrayed in the comics, in Marvel Fanfare #2. The writers also considered an episode for the series that would've had the Avengers as dinosaurs, but rejected it for being too silly.