There are some edge cases that would need to be solved, e.g. (with a grain of salt):

car, err := getCar()
if errors.Is(err, ErrNotFound) {
    return nil, err
}
car.SetDefaults()
return // What do we suggest here?

In this case, the user might still want to return err instead of nil but the err has been already returned once. E.g. the the variable has already been used in an earlier return statement most likely won't work. A lot of cases would be probably covered by checking for

car, err := getCar()
if err != nil {
    return nil, err
}
car.SetDefaults()
return // We can suggest `nil` for err as the error has been "completely" used

Not sure if that makes sense.

Anyway, I think the changes (if any) need to be made here : https://github.com/golang/tools/blob/master/internal/lsp/analysis/fillreturns/fillreturns.go