How to use Functor instances with Fix types

Let's say I want to have a very generic ListF data type:

{-# LANGUAGE GADTs, DataKinds #-}

data ListF :: * -> * -> * where
  Nil  ::           List a b
  Cons :: a -> b -> List a b

Now I can use this data type with Data.Fix to build an f-algebra

import qualified Data.Fix as Fx

instance Functor (ListF a :: * -> *) where
  fmap f (Cons x y) = Cons x (f y)
  fmap _ Nil        = Nil

sumOfNums = Fx.cata f (Fx.Fix $ Cons 2 (Fx.Fix $ Cons 3 (Fx.Fix $ Cons 5 (Fx.Fix Nil))))
  where
    f (Cons x y) = x + y
    f Nil        = 0

But how I can use this very generic data type ListF to create what I consider the default Functor instance for recursive lists (mapping over each value in the list)

I guess I could use a Bifunctor (mapping over the first value, traversing the second), but I don't know how that could ever work with Data.Fix.Fix?