Adv coc could already be beyond the "simple" sky split we have seen in WB vs Shanks or Kaido vs BM. That because the strongest clash in the manga is actually Roger vs Primebeard and the focal point of that clash is that "the blades are not even touching" and so the adv coc. Yet to reach the yonko lvl I agree Oda could very well show a sky split moment for Luffy too.. That could happen with the final attack of Luffy vs Kaido or be the greeting (because as we have seen until now that attack is just the greeting between 2 yonkos, it is like a thing they do to show each other they are "yonko lvl" fighters) when Luffy fights anothr yonko (and that means he finally has reached the lvl). Anyway if I have to bet what it is, probably it is not coa, it could be very well coc (maybe like the "echos of coc" of BM who were knocking out people on other floors, this is enouh to move clouds kms from the users) or even simply an extreme amount of strength (just that) that in the end only the yonkos have and display.