I think Luffy too OP too fast. There’s still awhile to go and now the next villain has to live up to being a threat to him and he just beat the strongest beast in the world.
It isn't like he's dominating Kaido, even with Gear 5, Luffy is barley holding on in this fight. And regardless of this transformation, from this point forward Luffy was only ever going to be fighting Emperor and Admiral level guys. That's where we are at in the series.
And do you really think that Shanks or Blackbeard is going to be any weaker than Kaido? And Big Mom will be a 1v1 fight. That's why Oda didn't let Kidd and Law KO her. And Sakazuki is going to be a tough fight. And I think with Luffy's power up, almost guarantees that Im is going to be a strong fighter, if not the Elder 5 as well.