Nearly everything that was done with Big Mom here, with the sole exception of her clash with Kaido, with Oda treating and using her terribly, nearly-always nerfing and handicaping her and removing her as an antagonist in the most horrible and retarded way possible while refusing to treat her as well as Kaido in terms of portrayal, as well as the waterfall and amnesia plots.
The whole Nika retcon, from Luffy's devil fruit being changed to this sun god prophecy mess, with zero build-up and foreshadowing before Onigashima, to G5 and his too cartoonish gags and laugh.
Kaido's backstory and motives, as well as his relations and dynamics with his crew, especially the Calamities outside of King, and Xebec, being so underdevelopped, as well as Orochi being the mastermind and most active of the two as antagonist.
Yamato and her loony obsessive fangirlism of Oden, as well as Oden himself and his retarded dance and decision to kneel to Orochi and Kaido.