A great plot for Greys Anatomy fan fiction could be a medical mystery that stumps all the doctors at Grey Sloan Memorial. It could involve a rare disease that has never been seen before in the hospital. Or perhaps a story centered around a character starting a new, revolutionary medical treatment program within the hospital, facing all kinds of opposition from the traditionalists. Also, a love triangle between new characters added to the roster could add some drama.
One idea could be to explore the backstories of minor characters more in - depth. For example, the lives of the nurses or interns who don't often get the spotlight. Another could be a time - travel plot where a character goes back to the early days of the hospital and tries to change certain events. Maybe also a story where two characters who are always at odds have to work together on a life - or - death case and end up understanding each other better.
Well, you could write a fan fiction where a character has a near - death experience and then has some sort of supernatural ability afterwards, like being able to see the auras of patients and tell how sick they really are. Another idea is to write about a big natural disaster hitting Seattle and how the doctors of Grey Sloan Memorial have to deal with the influx of patients under extremely difficult conditions. And then there could be a story about a character who discovers a long - lost relative who also works at the hospital and how that changes their dynamic.