Well just because OP went on a lot longer than it is supposed to doesn't mean it has to suck. If you replaced Kuma with Mihawk in the latest arc, suddenly everything might actually make sense. The latest arc complete defies the notion that Marines and Pirates are always fundamentally opposed to each other, and that this inseparatable difference is not something bridged by honor or friendship. While Kuma is not a Marine, he is about as close to one as it gets and his actions make no sense from someone who is supposed to be 100% loyal to the World Government. Some people say maybe he's a spy from Dragon, but then that's totally a Deus Ex Machina thing, and if you need that you could simply have Dragon appear from nowhere to bail the Straw Hats out. The deal Kuma made would be more beliveable from someone who has no loyalty in the World Government, like Mihawk. Even Moria would be more likely to accept such a deal.
The notion that Sanji or Zoro could be as important as Luffy is also utterly ridiculous. I thought it was downright insulting how Oda was trying to say that there is a member in the Straw Hats that is as important as Luffy. Even the ransom levels suggest Luffy is roughly as important as rest of the Straw Hats together, and the rest of the Straw Hats probably would have died 25 times a piece if Luffy wasn't around.
The author of Dai-no-daiboken says if you care for your characters, your characters will write themselves. Sora personifies hope, Leona personifies love, and Pop is courage. Courage always stands next to the hero. Hope shines eternal when all appears to be lost. Love will heal the land. If you need hope you wouldn't go talk to Leona. If you need courage you wouldn't talk to Sora. Even if the author said today Leona will replace Sora as the hope character, it still wouldn't work because these 3 are basically the virtues themselves.
In the Kuma arc, effectively one of the character, either Sanji or Zoro, wrote their own (possibly both) death. You say that Zoro personalifies sacrifice, and a sacrifice must be made for the survival of the Straw Hats. But the author forcefully intervened against the very qualities his character embodies. This is poor design.