There are lots of characters over many books that I'd like to read about. Be it a back story, or a story about their life later on. This list is in no particular order.
1-4. The Marauders: James, Sirius, Lupin and Peter. I know I cant be alone in this, I would love to read stories about their time at Hogwarts. I'm sure there's some Weasley twin level shenanigans and stories that can be told.
5. Peeta Mellark: I'd really like to read scenes from The Hunger Games from his perspective. Maybe what was going through his mind during the first games, or his time as a Capital prisoner.
6. Nikolai Lantsov: I may have wanted Alina and Mal together in the end, but Nikolai is still one of the greatest characters ever. I want to read stories about his time as a privateer.
7. Logan Kade: This is a secondary character in the Fallen Crest series that I really want to know more about. He has a decent role in the book, but I'd just really like to hear things from his perspective.
8. Isabelle Lightwood: She is sassy, strong and feminine and I think a pretty under appreciated character. I want to know more about her life before Simon and Clary.
9. Roth: From the Dark Elements series. I love his character. He is snarky, and sweet and funny.
10. I want more Day and June: With how things ended in Champion, I want to know where their story headed from there. It was a sad and sweet ending, and I want to know... I need to know.