Breha Organa was the last Queen of Alderaan, we could learn more about the history of the planet, the rituals around the Day of Demand (in which Breha lost many of her vital organs in her Challenge of the Body), how she met Bail...it would be great!
You can read more about her than in possibly any book in "Princess of Alderaan", snippets in "Queen's Shadow", and a short story from her POV in "From A Certain Point of View". It's the POV right before she and her planet blows up, so maybe not the best introduction.
There's Tel Angor, a former Jedi who, because of experiments by one group (The Silver Jedi), turned instead into a being corrupted by the Dark Side, and turned, essentially, into a Force Vampire. The Silver Jedi are news to me, so we can learn about them too! Yay?
The Father, the Son, and the Daughter - The Ones - are mentioned in the Mortis Arc of The Clone Wars.
They're basically the Big Gods of the Star Wars universe, who hang out on another planet and try not to interfere...until Anakin, Obi-Wan, and Ahsoka crash onto the planet. It's a great arc, so I won't spoil it. You can read about them in a few guides about the franchise.
But since there was a Father, Daughter, and Son, was there a Mother? Yes...and she is possibly the most badass character to ever be written in the Star Wars Universe.
"Mother" used to be a nameless woman who loved to serve The Ones on their lonely planet, and pretended they were a family. As she grew older amongst their youth, she drunk from some special spacial Force pool which corrupted her, and turned her into Abeloth, and immortal, evil being prominent in both sides of the Force.
She ruled planets, changed her appearance, forced people to depend on her by manipulating their emotions, fought Luke Skywalker...all to mysteriously 'die'. A lot of her stories are now Legends fodder, but as Clone Wars doesn't mention her, it's possible for her to return.
A side character in Claudia Grey's Master and Apprentice is Pax Maripher - A character who was orphaned by pirate raids and raised for years by protocol droids as a child. Wouldn't that be great to read about? We can also involve Rahara Wick, another character in the book, if you want to make it a Lost Stars POV switcheroo kind of thing.
Cad Bane has a loooot of prominence in The Clone Wars show (See a pattern here?). He's probably the most prolific bounty hunter (besides Fett) in the series. Not a lot of stuff about his past, or not interacting with the Jedi (Usually Obi-Wan), so a stand-alone adventure would interest me.
There is one more character whose name just TOTALLY escapes me. Her master was killed, so she lived alone in either a spaceship or a jungle planet as a Padawan for years. It's not Asajj, it's a human girl, in the comics. She also could totally use a book.
Update: Yeah I was actually more correct than I had thought - Maris Brood was both raised in isolation on a spaceship and lived on a Jungle Planet with Shaak Ti (Who could also use a book!) later before turning to the dark side.
Look forward to the prequel to "Queen's Shadow", "Queen's Peril", about Padme first becoming Queen of Naboo, to be released in early May.
sources are in the post.