With the Star Wars trilogies having a clear agenda for the story they were telling, there wasn't much time to introduce all the characters that are seen, let alone give them a back story. The animated series Star Wars: The Clone Wars was able to give some of those characters, and more, some much-deserved screen time.
However, even in the show, there are some characters that are more than deserving of getting their story told more than it was. From of the Jedi Council to outlaws that seem to play both sides, these characters in Star Wars: The Clone Wars deserve their own show.
Shaak Ti
The Togruta Jedi Master, Shaak Ti, is often seen in either Jedi Council meetings, or on the planet Kamino alongside Nala Se helping with the training of the clones. Her wisdom and patience are what made her a strong and unique Jedi.
Both on and off the battlefield she showed just how powerful she was. Her time on Kamino alone could prove to be its own show. Showing fans how involved in clone trooper training she was, as well as her internal battle of separating her comion for the clones as living beings versus them being produced to be well-trained clones for battle.
Nala Se
Nala Se is one of the head Kaminoan doctors and scientists at the clone production facility on Kamino. She is clearly very smart, and comes across as calm, cool, and collected. In The Clone Wars, she tries to hide the inhibitor chips from Shaak Ti when Fives tries to prove that's what made Tup turn unhinged.
Her intentions are questionable (this is proven even more in The Bad Batch) at best. Seeing a show on her and her development of the science of creating the clones and how she was pulled into everything would be a great backstory for a series.
Plo Koon
Jedi Master, Plo Koon, can be seen in the prequel Star Wars trilogy, as well as in The Clone Wars, where he has a much more prominent role than he does in the live-action trilogy. Plo Koon is not only a powerful Jedi Master, and member of the Jedi Council, but he is also the one who found Ahsoka Tano and took her in.
Seeing his connection grow with her, finding out why she did not become his Padawan, and how that shaped Ahsoka's story arc to be unexpected in the show, would be the story fans might be looking for on him. While he may be a quiet member of the council, his actions speak volumes, and when he does talk, it is always with purpose.
Cad Bane
There are many bounty hunters that show up for their own nefarious purposes in The Clone Wars, but Cad Bane may be one of them that stands out the most. Bane has a certain flair about his character that makes people both respect and fear him at the same time.
He is defiant, confident, and known to be one of the best blaster shooters around. He gets a few moments in the light showing he knows how to work with others, but also works alone sometimes. Learning more about him in a show, how he became a bounty hunter, and his role in training Boba Fett would be a Star Wars fan's dream come true.
Asajj Ventress
With the introduction as a Sith Padawan, Asajj Ventress' story arc turns into quite a rollercoaster. As Ventress might be one of the most tragic characters in the Star Wars franchise, she still has a lot to prove. While most of her story is told in Star Wars: The Clone Wars, it's the tale of her life as a Jedi Padawan, and before that would make the fans excited.
With the flashback that is seen in "Nightsisters" of her being a padawan only for her master to be killed, and her to be picked up by Count Dooku, it shows she has a much deeper backstory than is shown. Learning about her past could help fans to understand why she seems so lost in her journey in The Clone Wars.
Mother Talzin
The Clan Mother of the Nightsisters, Mother Talzin, may just be one of the most powerful figures in the Star Wars franchise. As the Nightsisters got some screen time in The Clone Wars, it was truly just the tip of the iceberg of their whole story. With their dark and mysterious powers, that seemed to be connected to the Force, it begs the question of their connection to the Jedi.
Mother Talzin alone could not only hold her own show diving into her past, the past of the Dathomir people, and the development of the Nightsisters but could drive the beginning of a show about the different cultures in the franchise.
Luminara Unduli
Luminara Unduli is a notorious no-nonsense Jedi Master. She gets straight to the point and it shows in her interrogations, as well as in her fighting style. She trains Padawan Barris Offee, who betrays not only her friend Ahsoka Tano but also the entire Jedi Order. Luminara and Barris are both seen in the prequel trilogy, but their story gets told a little more in the show.
Seeing as Luminara is a great Jedi Master, knowing more about her relationship with her Padawan and how she felt when she found out about the betrayal would make for a great story. With how little Luminara's character is shown in action, versus how much she is in the background of shots, it would be great to give her the spotlight she deserves.
Hondo Ohnaka
The infamous pirate and outlaw, Hondo Ohnaka makes many appearances in both The Clone Wars, as well as Star Wars Rebels. He plays both sides depending on which benefits him more, but ultimately can be seen helping the good guys in pivotal battles. There were many times that he proved himself to be a very clever leader, as he was the leader of the Ohnaka Gang, but he was also one to recognize and it when he had been beaten, which made him surprisingly humble.
Seeing the balance of good and bad constantly ebbing and flowing in one character who is so charismatic makes him one of the most underrated characters in the series. Seeing Hondo's life before, as well as all the bits fans didn't see during the time of the Clone Wars would make for a great show.
General Grievous
Perhaps one of the most underrated and scariest villains of the Star Wars franchise, General Grievous has a story as to how he ended up as he did, and fans want to know.
Making only a brief appearance in the Star Wars prequel trilogy, it's not until fans see his lair and learn that he has lots of spare parts to fix himself whenever he is damaged, that they realize there is quite a story to this villain. General Grievous' past would make a great movie, as it would give some insight as to what happened to him to make him mostly robot because, under his helmet, he's clearly an organic being.
Captain Rex
One of the most well-known clone troopers, Captain Rex, is one of the few to make it out alive through the Clone Wars as well as survive Order 66. Having fought alongside some of the most notable Jedi, like Anakin Skywalker, Obi-Wan Kenobi, and Ahsoka Tano, he lives a very adventurous life.
Most of his life during the Clone Wars can be seen in the animated series Star Wars: The Clone Wars, but it's the time between when Order 66 gets initiated and when the Phoenix Squadron runs into him, Gregor, and Wolfe that would be interesting to see. With how close he was to all of his brothers, seeing them all turn against the Jedi must have taken a big emotional toll on him, and it would be a good film to see his journey - both emotionally and physically.