There are too many villains that need to be defeated, and going by story progression I think that Luffy/SHs will likely have only two major villain groups now. No matter how much the fans want Luffy to face off against Shanks, I think the chances of that happening are almost zero. There is not much left in terms of story progression and subsequently stronger villains. Any combination of order of fights Shanks > BB > Akainu does not make sense. At best you can only have one of these three as the next villain and then the final fight against Gorosei/Im. You can say that Luffy is not Kaido level yet because of the many chances he got against Kaido and hence he needs one opponent to get to or beyond that level and that opponent is most likely one of the three mentioned above.
In terms of abilities though there is not much left, he already has all types of haki with advanced versions and DF awakening. The most you can say is that his haki usage, pool and strength can be improved, e.g. Shanks can see 10 secs in the future and Luffy is probably doing less than that. So even if you somehow find a way to cram BB, Akainu, Gorose/Im as antagonists, how each of them is defeated by using a new powerup is not possible. Specially when it comes to Akainu, Luffy already showed his way of dealing with magma with the no touch method against Kaido.
In terms of abilities though there is not much left, he already has all types of haki with advanced versions and DF awakening. The most you can say is that his haki usage, pool and strength can be improved, e.g. Shanks can see 10 secs in the future and Luffy is probably doing less than that. So even if you somehow find a way to cram BB, Akainu, Gorose/Im as antagonists, how each of them is defeated by using a new powerup is not possible. Specially when it comes to Akainu, Luffy already showed his way of dealing with magma with the no touch method against Kaido.
