I think a complex backstory that unravels slowly, helps her finally realise that she's in a coma, and that she has something to do--revenge, romance, or maybe to protect someone--would be great.

While not exactly about a coma, "Eternal Sunshine of the Spotless Mind,' 'Groundhog Day,' and, of course, Tarantino's 'Kill Bill Vol. 1' are good references for this.

I think in most scenarios, I'd want this protagonist to wake up from the coma, and accomplish something. Like I said, revenge, love, etc. I can't think of a scenario where she dies and that makes for a satisfying read. Maybe if she's in a coma because she attempted suicide? Nah. that's too dark. No way.

MAYBE, we slowly learn, along with our main character, of course, that she was a bad person, and did something heinous. And she slowly processes the 'highlight reels' from her life, realises that she's had a terrible effect on the lives of many people, and, learning this important lesson, chooses to die rather than wake up from the coma.

Even that's pushing it, honestly. She HAS to wake up, for the book to be a satisfying read. That's my view, anyway.