The new era of Star Wars under the ownership of Disney, and without the involvement of the maker George Lucas, has been great for the most part, and, while divisive in many ways, has created a new canon with so much brilliance throughout it. In this new canon, the movies and shows have obviously stayed intact, while the whole other world of the Expanded Universe no longer exists.
The Expanded Universe, now known as Legends, had a lot of nonsensical garbage, and a lot of narrative brilliance, as well as some genuinely excellent characters. With the timeline no longer being real and recognized, so many characters are lost.
Prince Xizor
In Prince Xizor, the Expanded Universe, or Legends, has a crime lord as powerful and ambitious as any other, with charm and intellect to him akin to the likes of Thrawn.
He is a character whom many fans of the EU/Legends have wanted a return from, and if he can somehow get brought into the criminal fold in some form different from his original role in the likes of Shattered Empire, it may be worth the trouble.
HK-47
Most of the central characters of the beloved and exceptional Knights Of The Old Republic video game no longer exist in the canon, like Darth Malak, Carth Onasi, Bastilla Shan, and the murderous HK-47.
The adored assassin droid is the source of so much hilarity in the Old Republic era with his hatred of meat bags and skill with weapons. He is one of the game's best characters, and fans hope he, along with the likes of Bastilla and Malak gets brought to the canon.
Natasi Daala
Natasi Daala's role as a strong, fierce, beloved female who is high ranking in the Galactic Empire is one currently occupied by the excellent Rae Sloane. Still, the Admiral definitely is a character who could join Sloane, who unfortunately does not exist.
She is a terrific strategist and a barrier-breaking female in the organization. While characters like Sloane show the ability of females to be equal to that of men, having Daala brought in could very much further this cause as well as just having another great Legends character as canon, but the Empire is very discriminative.
Trioculus
As mentioned earlier, the EU had some excellent characters, as well as some utterly brilliant stories that fans do miss. But it also had so much awful nonsense too.
That is where Trioculus comes in, a three-eyed mutant who usurped the position of Emperor for a year, claiming to be the son of none other than Darth Sidious. His story was terrible, and content like the story of Darth Vader's glove is best left in the past.
Galen Marek (Starkiller)
The Force Unleashed was a fantastic Star Wars game that, unfortunately, does not fit into the new canon in any which way, removing any possibility of meeting Galen Marek, a.k.a Starkiller as we know him ever again.
The apprentice of Darth Vader was a great character and one who was immensely enjoyable to play through in two different video games in 2008 and 2010, where he got fleshed out a surprisingly good amount for someone who was just a video game character in the Galaxy.
Ajunta Pall
There is a strong possibility Ajunta Pall does exist. He does in legacy, with the name of his canon counterpart merely unknown. Unfortunately, without confirmation of the name, Ajunta Pall remains in Legends purgatory.
Pall was the original rogue Jedi, who split from the Jedi Order, sparking the One Hundred Year Darkness. In canon, there is only an unidentified rogue with little information on the darkness, leaving Pall behind in the Expanded Universe.
Kyle Katarn
The Expanded Universe past the events of the original trilogy more so extended the history of known heroes, while creating original villains. Kyle Katarn, though, was one of the most beloved and original heroes of the Expanded Universe.
His role in the Jedi Knight games is terrific, and he is a hero many fans who know of him or the games have desperately wanted back in the canon. Katarn had a strange life, switching between the Light and Dark as well as his general role in the Galaxy quite a bit. He still remained terrific, however.
Darth Nihilus
One of the things the Legends timeline has over the new canon in spades is badass Sith. In all honesty, there are perhaps too many. It's like every Sith is so powerful it just detracts from their characters entirely, whereas the deep-dive exploration of just Vader the canon is going through simply makes him better and better.
Some iconic Sith have made their way to canon, like Revan, Bane, and Exar Kun, however, some remain lost. One of the most notable is the incredibly fascinating Darth Nihilus, a Sith Lord with immense power who would make a fine addition to the canon alongside Sith like Darth Malgus.
Mara Jade
When a sequel trilogy set decades after the original trilogy got announced, many fans prayed that Luke Skywalker would get joined by Mara Jade, his wife, and the mother of his children as well as former Hand of the Emperor.
She is a character fans could not get enough of, and there are few who fans have universally wanted to get brought into the fold. At this point, chances of that happening are slim to none, which is truly sad.
The Skywalker/Solo Children
While there is a slim to none chance of Mara Jade ever joining the canon, there is precisely zero chance of the famous children of her and Luke's as well as those of Han and Leia to exist ever again.
Ben Solo/Kylo Ren is the excellent character tasked with filling the void of Anakin, Jaina, and Jacen Solo, and Ben Skywalker. They were vital characters in a plethora of stories, and for the most part, were good characters. They do make us wish there were more children from the original trio, but Ben Solo is still a brilliant addition to the canon, and hopefully, the in-universe future is safe in the hands of the likes of Rey.