I generally can't do fanfic at all. My brain just... doesn't seem to work that way?
I've written (I think?) a total of 3 fic in my life.
Before I started really writing (or had ever heard of fic) I used to come up with fic all the time, I'd fall asleep telling myself stories of what my fave characters got up to after the series was over or whatever.
But for at least the last 20 years... my brain won't go there.