Paul Walker?

I heard a while ago that his brothers were interested in having the character back in the series, but we won't know until 2021. I do feel that the return of established characters like Mia, Han and Sean could fill his place without replacing him, though Sean wouldn't necessarily be part of Dom's inner-circle.
After F8 was released I always thought that Little Nobody (Scott Eastwood) could fill his place for the next two films, but now that seems unlikely unless he appears in a small role in F9, but I wouldn't rule out an appearence in the tenth film. I didn't mind Eastwood's character in 8, his bickering with Roman was fun and he fitted in well with the team, but still he's no Brian.

Unless they plan on using Walker's brothers to reprise the character, they should have given Brian some sort of heroic death in F7 and continued the series without him. It's not believable that O'Conner would sit at home off-screen babysitting while Dom and the rest face their life threatening adventures. They use the excuse that he is doing it for the baby, but that doesn't excuse Mia getting back in the game. Plus, it will make Dom look like a bad single parent since he will be out there saving the world with his baby from F8.