I Knew It: Almost immediately after Lucasfilm announced that the old Expanded Universe had been Exiled from Continuity and rebranded as Star Wars Legends, fans of Ensemble Darkhorse Grand Admiral Thrawn started insisting that Thrawn would show up in the new Expanded Universe eventually. They were absolutely adamant that he was still around, even though Timothy Zahn's The Thrawn Trilogy—where he was introduced, and later Killed Off for Real—had officially never happened. But in 2016, it turned out that they were right after all: it was announced that Thrawn would appear in Season 3 of Star Wars Rebels, and that Zahn would be penning the new novel Star Wars: Thrawn, which would officially reintroduce everyone's favorite Chiss in White with a new canon backstory.
Rebels is quite fond of reusing old concepts, such as the Bendu (the original name of the Jedi was the Jedi-Bendu; Yoda was also originally going to be a gigantic creature, which is what Bendu is), Ashla and Bogan (the original names of the Light and Dark sides (also reused in Legends' Dawn of the Jedi), character and place designs, and so on.
They also reuse concepts from more recent times too such as The Force Awakens, like the Fifth Brother's design being based off of an unused Knight of Ren design and Ketsu's helmet being one of Kylo Ren's concept art helmets.
The show's focus on a crew is based off the original concept of The Clone Wars, where a Jedi master-apprentice duo would be traveling with a smuggling crew during the war.
Saw Gerrera was originally supposed to be in a live-action show (presumably Underworld). After the Disney buyout, an extremist character was needed for Rogue One, leading to Saw's original purpose being refitted.
What Could Have Been: One of the most famous is the proposed mid-nineties novel Alien Exodus. This was intended to reveal the origin of humans in the Star Wars universe, with a group of astronauts escaping a computer-dominated future Earth (supposedly the same as the one featured in THX 1138) and somehow ending up "a long time ago in a galaxy far, far away".