From the "Post Gains Doffy" series, here comes: "Post Gains Katakuri" lmao.
Doflamingo will get power bumped because he's popular and because Oda doesn't care about scaling as much as the fans. As in, he needs Doffy to be relevant when he returns, so he'll be able to hang with stronger characters. No real reason other than that.
Katakuri has good reason to be stronger. He's been opened up to failure and a whole new world to experience and a new perspective. All from WCI alone. Factor in Linlin's dubious continued existence, the fact Cracker was frozen solid, unnamed members of the family beaten by Germa, Pudding kidnapped, etc. Katakuri's ambition to keep his family safe is being tested.
And he's going to god damn thrive in this adversity. Katakuri vs Kuzan is the sleeper match up you guys will thank me for, in 2 years.