Michaela Pratt - How to Get Away with Murder
Michaela is probably the standout character of How to Get Away with Murder, aside from Annalise Keating, which is due to Aja Naomi King's talent and the character's history. Her devotion to being a lawyer, and being the best, would be interesting to explore if she was written off onto her own spin-off that starts out with her career first beginning after law school.
Daisy Johnson - Agents of SHIELD
Daisy may be considered the "main character" of Agents of SHIELD, but does she really fit with the team anymore? The premise was originally ordinary people being part of SHIELD and fighting those with special abilities, but with Daisy, it has become more of a powered people agency. Daisy should have been the star of the Inhumans show with a different premise... at least, then, it might have lasted.
Katherine Pierce - The Vampire Diaries
Katherine never got the attention she deserved on The Vampire Diaries due to Nina Dobrev playing two characters and not having enough time in a day to play two characters all the time. Katherine was interesting, cunning, and certainly she had an interesting life. It'd be awesome to see flashbacks to her life in another era, if the producers and Dobrev ever chose to do so.
Mulan - Once Upon a Time
Mulan is the LGBT+ hero we need on television, and I will forever be bitter she isn't the star of Once Upon a Time Season 7's new direction. Mulan never got the attention she deserved, and had her heart broken far too many times.
Trevor - The Good Place
Trevor's appearance may have been short, but Adam Scott's great performance led to an outstanding character, although he's a total dirtbag. It'd be interesting to see him in a spin-off about his "bad place" and how he's torturing these humans. Who knows... maybe he can change, just like Michael?
Trish Walker - Marvel's Jessica Jones
While it's amazing to see the friendship between Trish and Jessica, Trish has the potential to have her own great spin-off series following her adventures as (comic book spoiler) Hellcat. She's a badass, but on Jessica's show, she's never going to get the focus she needs to truly flesh out her character and the hero she'll become.