This page lists planned episode concepts (mentioned by the Crewniverse) that were never made into actual episodes of the series.
Gem Mythology Episode
This episode would have taken place before Peridot joined the Crystal Gems, but after her truce with them. It was about Peridot discussing the mythology of the Gem Species.
"The Show"
This was a planned episode involving Pearl and Mystery Girl at a roller rink (as well as including Steven and Amethyst). Not much is known about this episode concept.
Rhodonite Episode
This was a planned episode in Steven Universe Future. It would center on Rhodonite. It was scrapped due to it not being relevant to the overall plot of the show.
Sources
https://www.vulture.com/2020/03/steven-universe-future-finale-rebecca-sugar.html