Honestly? I kind of thought it'd be more of a plot point than it really turned out to be, at first.
I thought they were being mind controlled. Especially since they returned in the same arc where we learned about the World Government having clones of the Warlords. I figured Oda was going to reveal that the World Government was mind controlling some high-level fighters against their will with some sort of technology, maybe from the Void Century or something. Something the Strawhats would have to break them free of. Maybe have someone like Koby end up going through the same brainwashing program, in order to turn him into a bigger threat later on in the story.
But...nope! The line in their cover story about them returning someday wasn't a threat, like I assumed. It was just them telling the World Government that they were just going "on vacation" for a while, as it were. I guess this is really MY misunderstanding. Or a slight mistranslation, at worst, perhaps.
But, CP9 becoming CP0 offscreen in a adventure we only ever heard about via a SBS is a bit of a disappointment, I'm not going to lie. It felt like a bit of a anticlimax to a character arc that really only ever existed in my head. Still, bit of a letdown.
I do feel like Oda dropped the ball a bit with this plot line. It could have been so much more. Or even so much less, if Oda had just given us a panel explaining why CP9 was back with the World Government again when they first showed up. Instead, Oda treated it like a mystery. A mystery we didn't even get to hear about until it was solved in a SBS. But, Egghead did that a few times. So, I wonder if something was happening behind the scenes. Maybe Oda just got distracted with the live-action show and forgot a few of the mysteries he had set up. But, either way, I don't think it was handled the best.
I do wish we had at least gotten a bit of a character focus for why they went back, though. A "maybe it's better to stick with what we know" sort of moment. A quieter moment when CP9 resign themselves to the idea that they really aren't suited to be anything other than killers. A moment where they all talk through this choice, before they make it. That would have been nice to see. Even as a brief flashback. But, alas, it was not meant to be.
I really don't mind the idea of CP9 rejoining the World Government. But, it does feel a little regressive, the way Oda did it. Like, they're just being villains again, because Oda can't think up any other way to bring them back into the story. Just reverting the characters back to who they used to be, rather than letting them grow into something else. But, at the end of the day, this story isn't really about CP9. So, I can't really blame Oda for not giving a lot of precious page time to characters who are basically just side-characters, at this point.
It's not ideal. But, I guess I'm willing to let it slide.
