The formula for one piece right now(and for a long time) is not characters. Oda knows his limits I would say. He can't handle character and its a long running shounen. He doesn't want to take risk of potential plot holes and inconsistency(there already are though) if he tries to do something because these things most probably oda has not planned at all.
From what I see, his formula is create hype and show case....basically have 10 mid setup chapters at best then show one panel of new info which makes readers hooked and the repeat the process. Anime does have to deal with that shit too so they try to add some depth but we feel its dragging a lot(yeah its dragging too but I don't think they have much choice, the original content is not good enough tbh now compared to early days), like you can see how well Anime did with the Hype moments with Introducing Imu much better than oda did in manga.
And I won't blame him on that too. As I think right now his main objective is to finish the series while printing money. If one piece was not selling well...we probably would see more effort put in the series by oda and editors too.
For that right now he is mainly concentraing on Luffy the MC to have more screentime(though people are not liking it for obvious reasons) and the only focus.
Other than that he tries to write story around side characters to try to keep the story telling going on and interesting which is again just a hit and miss.
Unlike initial days where its easy to manage plot, story and characters. He can't do it now or wont prioritize it. Plot will proceed, story may not be present and he has to sell Luffy as the MC as end is near.