Paul Walker had every intention of continuing with the Fast franchise, and even though we can’t know exactly what he’d want, the people closest to him know how much he loved the Fast Family.
There are any number of ways they could do that tastefully, especially if he’s just in shadow or seen from the back.
They don’t have to make it creepy.
If the character of Brian O’Conner doesn’t appear in F9 after all, you have to imagine they’ll find a way to add him to Fast 10, especially if it is the final movie in the saga.