Tatooine was envisioned as a jungle world until Lucas decided he hated jungles while searching for a suitable location and made it a desert world. Tatooine used to be a jungle planet until it was Base Delta Zeroed by the original Evil Empire, the Rakatan Infinite Empire because the native Kumumgha people told them exactly where they could stick it.
Starkiller used to be Luke's surname before Lucas decided on "Skywalker" instead. The Force Unleashed made it the code-name of Vader's apprentice.
The Reenlistment of BaronFel is sitting in comic book and novella forms on the hard drives of the writers, unsold.
To say nothing of the tortuous, on-again, off-again paths of Crimson Empire III and the Darth Plagueis novel, both of which have finally seen release.
Mandorla, the delayed and then cancelled Nomi Sunrider novel, as well as the Sword of the Jedi trilogy, were cancelled when Disney rebooted the Expanded Universe.
Executive Veto: LucasArts set several universe-wide rules for every writer who wrote for the continuity.
The 1994 Star Wars Style guide imposed several restrictions on writers. Changing the power structure of the galaxy or introducing new technology was a no go. One of the biggest rules imposed in it before the prequels came about and lifted it was "Do not talk about the past". Specifically, writers were barred from depicting anything that took place before A New Hope (i.e. the Clone Wars, specifics of the Old Republic, how the Emperor came to power, how the Rebellion stole the X-Wing prototype, the fall of the Republic and the Jedi Knights, the history of the Emperor and Vader, the history of the Mandalorians, and anything about the history of the Jedi Knights). The lone exception to these were The Han Solo Trilogy, The Han Solo Adventures and The Lando Calrissian Adventures books, which are prequels to A New Hope and Empire Strikes Back whose publications precedes the mandate.
Under no circumstances were Luke Skywalker, Han Solo or Princess Leia to be killed off. The only works that got around this were either stories set in the far future and thus beyond the natural lifespans of the characters (which by the end of Legends meant only Star Wars: Legacy), or non-canon Infinities stories.
Yoda's species and home planet were not to be named. Even the current Disney canon has to abide by this rule to this day. This was taken to such an extreme that a card illustration depicting many more members of the species worshipping a statue was recalled and pulped upon Lucas finding out about it.
There was a longtime ban on Darth Sidious' childhood and apprenticeship to Darth Plagueis, which was eventually lifted with the publishing of Darth Plagueis in 2012.
Wookiee Jedi and other Wookiee Force users were forbidden. This ban came a number of years into the series, so several Wookiees were protected by a Grandfather Clause.
An embargo on Hutt Jedi was instated, though not before the book Planet of Twilight had introduced the one-off former Jedi Knight Beldorion the Hutt who died at the end of it.
George Lucas forbade the Expanded Universe from directly depicting the Clone Wars or the fall of the Old Republic and the rise of the Empire until the release of the prequel trilogy (some material ended up needing retcons). Dark Horse Comics did, however, depict the ancient wars between the Jedi and Sith in Tales of the Jedi.
There was a longtime ban on Darth Sidious's childhood and apprenticeship to Darth Plagueis. The ban was eventually lifted with the publishing of Darth Plagueis.
Surprisingly Lenient Censor: As a Jedi Master, Soon Bayts' title is Master Bayts. Randy Stradley inserted him as a joke character in the Acts of War miniseries which preceded Republic and was surprised when the editor didn't seem to pick up on the pun.
A Return of the Jedi extra named Jess could have been made more than a background character during the first trilogy revamp, given a semi-documentary belonging to Warwick Davis showed a background dance practice with her and Laudica's actress's which might have been intended for Return.
In the original run Clone Wars novels, Shaak Ti was supposed to receive A Day in the Limelight with the novel Escape from Dagu; however, the story was cancelled and replaced with Yoda: Dark Rendezvous due to fears that Ti was too obscure for to be a central character. (Understandable, since by 2004 Shaak Ti had only appeared as a background Attack of the Clones character, as part of a Jedi ensemble in one Star Wars: Clone Wars episode and in a handful of comics.)
Gail Simone was close to successfully proposing a limited run comic about Leia a little after the Disney buyout (before the Legends rebranding and when the comics division was still under Dark Horse rather than returning to Marvel), but it was vetoed due to her story beats contradicting the EU (which one she means is unclear — either it contradicted what was already established in Legends or what Lucasfilm had planned for her in new canon).
A Star Wars: The Force Unleashed webcomic about Shaak Ti before the events of the game was planned, but was mysteriously cancelled alongside a Rookies webcomic continuation.
Likewise, around the time of the Republic Commando series, Star Wars webcomic illustrator Tom Hodges revealed that he would be working with Karen Traviss on a webcomic after Evasive Action and hinted it would be about Mandalorians. However, there was no further word about this, mysteriously.
A Boba Fett novel by Traviss was set for release in 2011 but was cancelled due to her departure and difficulties with what Star Wars: Underworld had planned for the character. The story would have been set after Legacy of the Force and had flashbacks to Boba's past, though it never reached the draft stage of the process, so virtually no plot was ever planned.
Traviss was also supposed to pen more books in the The Clone Wars tie-in book series, but her co-author Karen Miller headed the series herself after her departure from the franchise.
Mandorla was an Alex Irvine novel that would have been about Nomi Sunrider, the Mandalorians, and the Sith, taking place right before Knights of the Old Republic. It was cancelled in 2012.
The Sword of the Jedi trilogy would have been written by Christie Golden and been about Jaina, Jagged, Ben, Tahiri, and Allana, set after Fate of the Jedi and Star Wars: Crucible.
A Paul S. Kemp duology and a Troy Denning trilogy were planned, but cancelled after the Disney acquisition and Legends rebranding. Nothing is known about Kemp's duology, though Denning's trilogy was meant to take place after Star Wars: Crucible, alongside the also cancelled Sword of the Jedi trilogy.
In addition to writing the first draft for The Empire Strikes Back, Leigh Brackett was contracted to write a novel about Leia. This idea ended when she tragically died in 1978.
Jack Chalker was originally to write what became The Han Solo Adventures trilogy, but chose to finish off his own series instead.
Daniel Wallace was set to write The Essential Guide to Episode I, but it was scrapped once it was clear the hype machine around The Phantom Menace had ended. Much of the lore set to be revealed instead made it into other reference books, such as the line of New Essential Guides that came throughout the early-to-mid 2000s.
One of the most famous examples of "What Could Have Been" in the Star Wars franchise 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 (one of them implied to be descended from Curtis Henderson of American Graffiti) 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".
to successfully proposing a limited run comic about Leia a little after the Disney buyout (before the Legends rebranding and when the comics division was still under Dark Horse rather than returning to Marvel), but it was vetoed due to her story beats contradicting the EU (which one she means is unclear — either it contradicted what was already established in Legends or what Lucasfilm had planned for her in new canon).