So lets just assume that Kawaki really killed Naruto .I am gonna tell you what might happen next.The only and best theory that appears in my mind is that Sasuke will use rinne-rebirth to revive him.Now some people might say that he'll die if he use rinne-rebirth (not true) so for that they could plant rinnegan in Naruto and hence he'll revive Sasuke as he has more than enough chakra to survive that jutsu.In last both of them will go and help Boruto in beating Kawaki.