The iconic Star Wars has been a science-fiction phenomenon since its premiere in 1977 which took the world by storm. The universe that exists in "a galaxy far far away" has expanded through series, both live-action and animated, novels, comics, graphic novels, and pretty much every medium that channels pop culture.
The book series that directly connects to the Star Wars universe of the movies and Disney+ series are confined to the Canon line, but some important Sith/Jedi rivalries remain in Legends. However, the Star Wars Legends string contains over 50 novels and has characters dating back thousands of years before A New Hope. These characters, both Sith and Jedi, deserve their time on the big screen for their fantastic storylines.
Darth Bane
Originally born as Dessel, Darth Bane is introduced early on in the Star Wars Legends series. He was a very poor miner on the Outer Rim planet of Apatros with an abusive father. Bane flees the planet to escape the Republic after killing a Galactic Republic ensign.
It's no wonder Bane ended up in the Sith Brotherhood of Darkness. He created the Rule of Two, "one to embody the power, one to crave it." Thus, Darth Bane became a Sith Lord, destroyed the Brotherhood of Darkness, and sought out an ancient Holocron to uncover the secret to immortality. These events, accompanied by his badass orbalisk bodysuit, need to be brought to the big screen.
Darth Zannah
Darth Zannah was Darth Bane's apprentice and his second in the Rule of Two, considered an awesome Legends event that could happen in Canon. She studies Sith Magic, which is something audiences haven't gotten to experience through the current Star Wars film and television features.
Her origin story would be an awesome segue into a series or as a plotline within Bane's story. Zannah, only a child at this point, is found after the detonation of the Thought Bomb that kills the Brotherhood of Darkness by Bane where he witnesses her first kill. Her upbringing and subsequent defeat of Darth Bane has so many entertaining elements that fans would drool over in a film medium.
Darth Tenebrous
Stemming from the lineage of Darth Bane, Darth Tenebrous was a legendary starship designer throughout the Galaxy. Tenebrous's attempt to create a virus that would disconnect the Jedi from the Force, even though it was an ultimate failure, could be a full-length movie in itself.
Another unique element of Darth Tenebrous's story in Legends is that he developed a way to store his consciousness into what he called "maxi-chlorians" so that it could transfer into someone else, not unlike the Reverend Mothers of the Bene Gesserit in Dune.
Darth Plagueis
The apprentice of Darth Tenebrous, Darth Plagueis lived in the public disguise of Hego Damask II of the InterGalactic Banking Clan. He used his professional status to manipulate intergalactic politics and business from the inside. Plagueis's narcissism is reflected in his obsession with eternal life and midi-chlorian sciences.
From his secret political agenda in the interests of the Sith, it's no surprise that, in the Legends universe, Darth Plagueis is the Master of Darth Sidious, otherwise known as Palpatine. Needless to say, turning this plot into Palpatine's film-adapted origin story would not turn fans off and has been mentioned as a major Legends event that should be adapted.
Darth Revan
Revan is a Legends character who began as a Padawan of the Jedi and training in the Light Side of the Force and ended up a Dark Lord of the Sith. Bringing in the popular Mandalorians, Revan played a pivotal role in the Mandalorian Wars.
Audiences are so used to seeing characters go from the Dark Side to the Light, or being tempted by the Dark and ultimately resisting. This storyline would be a nice change of pace on the big screen by seeing the actual transformation of a Jedi into a Sith, outside of Anakin, of course.
Darth Nihilus
Darth Nihilus began his reign as a Dark Lord of the Sith following the events of the Jedi Civil War, which could be its own series, to be honest. He eventually became the Lord of Hunger because of his insatiable hunger to channel the Force and became one of the most overpowered Force users in Star Wars Legends.
After causing catastrophic devastation to the Jedi, and subsequently absorbing their Force, Darth Nihilus continued his effort to quench his thirst for power. His origin story begins post-Mandalorian Wars and his rise to Sith Lord would include intense action, betrayal, and an exciting new era in the Star Wars franchise.
Naga Sadow
Naga Sadow is one of the earliest characters in the Legends line, dating back to over 5,000 BBY (Before the Battle of Yavin). Sadow trained with the Sith and advocated to expand its influence, which contrasted with other members' ideas to contain their philosophies while taking over the Galaxy.
Even without actually appearing on the big screen, Naga Sadow's influences on the future of the Sith are very important in Legends. His body is entombed along with artifacts and talismans, which help those who later find them in interpreting the Dark Side of the Force.
Shira Brie
Also known as Lumiya, Shira Brie served the Galactic Empire and garnered the attention of Palpatine and Darth Vader. The versatility of her character makes her an intriguing element to any Star Wars storyline and would be an instant fan favorite.
Shira Bris is Force-sensitive along with having exceptional piloting and combat skills. Her missions as a spy are classic entertainment and throwing that in with Palpatine and Vader is a recipe for a great movie or television show.
Jacen Solo
Jacen Solo is the Kylo Ren of Star Wars Legends. The son of Han Solo and Leia Organa, Jacen Solo would later become Darth Caedus. He trained along with his twin sister under Luke Skywalker and was the target of many kidnapping and assassination attempts due to his lineage.
What makes Jacen Solo's story different and, in turn, worthy of screen time, is his turn to the Dark Side. After being held in captivity for over a year as a result of the Yuuzhan Vong War, Solo was relentlessly tortured, subtly but significantly weakening his will. However, it took many years and many events to slowly convert him into a Sith Lord.
Mara Jade Skywalker
Mara Jade Skywalker's tale includes her time as a smuggler, a hand to the Emperor, Palpatine's personal assassin, member of the Jedi High Council, and the wife of Luke Skywalker. She was praised by Palpatine for her precision and assassin abilities. Upon her Master's death, Mara Jade sought out to fulfill his last wish, killing Luke Skywalker.
While on her mission, she goes rogue and a great many things happen that change her path to the Light Side of the Force. This Legends plot lets fans see Luke find a partner and eventually have a son. Needless to say, Mara Jade Skywalker's character has a ton of content that would appeal to any Star Wars fan, both Canon and Legends.