While the Star Wars franchise celebrates 40 years today, many credit its lasting success to the work of the novelists, comic book creators and tabletop and video game makers who kept it alive when there were no movies or cartoons to be seen. While its cultural impact wouldn't have ever gone away entirely, the world that became known as the Expanded Universe let people explore the galaxy far, far away in many new directions.
The Expanded Universe includes a massive list of new characters, from other Jedi and Sith to new wielders of the Force, to simple bounty hunters and droids that make this world an even richer one.
When Disney purchased Lucasfilm, the nebulous continuity of the EU was officially made into "Legends," meaning those stories were no longer canon ... but, as Ahsoka Tano said in a prescient manner on Star Wars Rebels, "There's always some truth in Legend." Indeed, Rebels has made many of the "Legends" characters and stories into canon (Star Wars: The Clone Wars did as well) or at least brought elements of them, the most prominent example of which is Grand Admiral Thrawn.
While Thrawn may be largely considered the best character from that continuity, there are many others that would fit well into the modern canon, even if they had to be slightly altered. While some, like the original EU children of Luke Skywalker or Leia and Han Solo, simply won't ever fit in, there are plenty others that can and should still come into play ...
While Star Wars: The Clone Wars introduced the concept of "The Ones" as "Force gods" of the mysterious traveling planet Mortis, it stopped short of bringing in Abeloth. Joining Father, Daughter, and Son as 'Mother' in the Legends stories "Fate of the Jedi," Abeloth is a mortal who twisted herself into an immortal when she betrayed her 'family.' With the Ones for all intents and purposes dying in their Clone Wars arc, this would be a way to reintroduce the idea of these ascendant Force beings into canon. Abeloth in particular is a very powerful and evil character, but she could help once again play with how we view the Force in Star Wars.
A dashing (sorry) smuggler primarily known from the Shadows of the Empire multimedia project (a game, novel, comics, audio book, all sorts of fun stuff), the character has remained remarkably popular, despite essentially being Han Solo lite - smuggler without a care, pilot of his own modified Corellian YT freighter, known for his ability to get out of sticky situations, etc. In Legends, Rendar helped the Bothan Spies acquire information about the second Death Star and had many clashes with another great Legends character called Prince Xizor. His closeness in character to Han Solo is actually an advantage - you can tell stories with Rendar that would''t quite fit Solo and his overall arc. His YT-2400 light freighter made it into Star Wars Rebels last year, so maybe Dash isn't too far behind.
One of the stars of the X-Wing novels by Michael Stackpole (which are excellent, by the way), Tycho was an Alderaanian TIE pilot who defected after the Empire, you know, killed everyone he ever knew and loved. That'll turn a guy. Joining the Rebel Alliance, he then flew for the New Republic, leading an X-Wing contingent you may have heard of: Rogue Squadron. If we ever get into more of that 30 years between Return of the Jedi and The Force Awakens, Tycho would be a great legacy character to bring in, even as a possible mentor to pilots like Poe Dameron and Temmin 'Snap' Wexley.
Shira Brie / Lumiya
Whether or not you liked her as Lumiya, a Dark Lady of the Sith, Shira Brie still has potential. As an Imperial, she was an intelligence officer who at one time successfully infiltrated the Rebel Alliance. After nearly dying and, like you do in Star Wars, getting some cybernetic enhancements, she trained as both an Emperor's Hand and secret apprentice to Darth Vader, eventually becoming a Sith Lady all her own.
While that latter half probably wouldn't work, a loyal Imperial infiltrator with cybernetic enhancements could still be a cool character to see enter canon. In the Star Wars comics that are canon from Marvel, we've seen more characters with cybernetics, showing the prevalence of such things in that world and helping Shira fit in. She could make a nice foil for a post-Return of the Jedi Luke Skywalker as well, though her 'Lightwhip' in place of a lightsaber would be harder to explain.
An ancient Sith Lord and magician, Naga Sadow very nearly took over the galaxy during the "Great Hyperspace War" in the Tales of the Jedi stories. His use of Sith magic is what makes him so interesting - you can almost think of him as a cross between the Emperor and Mother Talzen of the Nightsisters. Extremely powerful, his overall power levels would probably have to be dimmed a bit for the modern canon, but seeing a Sith who also used Dark Side magic could, again, have a positive impact on how we view the Force overall.
Ah, we are so close to Revan being fully canon. In Star Wars Rebels this year, Dave Filoni and company made the Jedi-Mandalorian Wars, the kick-off event to how Revan's story was told in Star Wars: Knights of the Old Republic, into canon. Fans hoped that meant Revan, who was both a prominent Jedi Knight and a prominent Sith Lord who returned to the Jedi order, would be canon once more.
Here's the only problem with Revan: his story of redemption could diminish the story of Darth Vader/Anakin Skywalker's redemption. That was supposed to be a singular event - you let anyone else do that and suddenly it's not as momentous.
Still, his story was so cool, and his supporting cast, including Bastila Shan and Malak, and his links to the Unknown Regions, which have grown increasingly important in the Star Wars universe, are all great enough to find some way to bring him back into the fold. If nothing else, at least we've got a Black Series figure from Hasbro.
Look, Mara Jade is almost certainly never going to be the wife of Luke Skywalker in modern continuity. We can accept that. But this smuggler, Dark Side assassin and eventual Jedi Master is still worth taking another look at. The title "Emperor's Hand" was brought into canon this year, at least ever-so-slightly opening the door for Jade's return, though Dave Filoni denied any direct link. As the Hand, she was a Force-using assassin for the Emperor, one tasked with a very specific target: Luke Skywalker. They eventually trained together, fell in love and even had a son, though again, all of that would be very difficult to bring into canon.
However, Dark Side assassins have been all over the canon stories, and that aspect would be easy enough to cover. As long as you still made Mara Jade a complex character who battles between a sense of duty and an innate sense of right and wrong, it would be worth making her a character whose legacy is more than just Legend.