This is a Scooby-Doo fanfic.

In it, Velma has dessert get into her eyes and she loses her sight and it's implied she would've died if not for the medical treatment, but despite this, the dessert was shown to be perfectly edible; Shaggy, Scooby, and Fred ate it without any ill effects.

After losing her sight, Velma also gains superpowers because apparently the author believes in all that "90% of your brain" baloney and the dessert's "chemicals" have leaked into her brain and caused inactive brain cells to activate. However, Velma was already weird before the accident because apparently, Dinkley women can live into their hundreds.

Velma falls in love with this OC woman (Sue? I can't quite tell but my Sue senses are tingling) who's a secret agent named Cassidy "Angel" Williams who calls everyone "baby" or some variation on it and they're referred to as "mates". Velma feels a "pull" towards Cassidy (I'm not calling her angel because she's anything but) before she even knew she existed. Also, Velma is said to be 16 in this fanfic, while Cassidy's in her thirties.

Anyway, I think Velma's been character-replaced. My reasons for thinking so:

1.) Her freaky abilities.

2.) Cassidy is possessive of Velma to the point of breaking the arm of a boy she thinks likes her, but Velma doesn't mind. She likes it even.

3.) Over the course of the fanfic, Velma never does anything Velma-ish. She does a tiny bit of investigating, but that's it. She doesn't read, she doesn't enthuse about science, she doesn't act particularly rational or intelligent, she doesn't say 'jinkies', nothing. All she does is get into the accident, complain about the accident, then get all smoochy with Cassidy.