So much for Luffy having his "Sky-splitting moment" against Blackbeard. Luffy is pretty much Yonko level at this point. I don't know why Oda gave Luffy such a massive increase in power. I mean where's the stakes against future opponents? It feels like Oda is rushing to finish One Piece. This arc is also going to end soon. The only major fights we have left are Raizo vs Fukurokuju, and Sanji/Zoro vs Queen and King. Also Big Mom, but I'm of the opinion that she won't go down in this arc.
Well, for starters what are you looking for?
- beating one Yonkou after much help doesnt cement you as Yonkou level in public eye also in all reader's eye
- if it's stakes you're looking for, i can say for like a hundred percent certainty that Blackbeard will be stronger than anyone in this world. And Imu will be even stronger.
- is it a massive increase in power that you're unsatisfied with? Well imo on the contrary it didn't even make sense that Rayleigh didn't even say anything about his captain (Roger) being able to imbue his attacks with AdvCoC. Luffy is a CoC specialist, all his efforts are converged on his "will to stand above others, impose his will on others" (CoC). If Zoro get CoA specialization due to his affinity towards strengthening his sword, it's a deserving powerup.
Same as AdvCoC for Luffy, Luffy trains his CoC the most since timeskip since it's his specialization. It's two years and Luffy didnt even train anything else other than his haki branches, different from Zoro who also trained for sword techniques aside from haki branches.
If only Ray didnt have amnesia about Roger's AdvCoC, Luffy would have figured out the theory of CoC Coating since the timeskip began, having much time and luxury to master it between arcs. Resulting in complete mastery since WCI or since Wano.
And that would be way too powerful for some, since we're gonna be so sure that he will not be stopped by anyone other than Yonkous and Admirals since the time of his mastery.
But Oda decided to make Rayleigh amnesic about his own captain's ability, and tone down - restrict Luffy's powerlevel until he defeated multiple times by Emperors, with haki bloom as an excuse. It's a way shittier storytelling in current scenario, since Ray's teachings didnt make sense.
But the more coherent storytelling (Ray telling Luffy about Roger's AdvCoC - Luffy training it like crazy between arcs to figure out the mechanics - Luffy oneshotting Hody Jones, two-shotting Doflamingo - threeshoting Cracker and Kata, stalemating Big Mom due to his talent in learning) will result in Luffy being Yonkou level since start of WCI or Wano right away...and i'm not sure thats what you want.