@King-Cannon
I lean towards the "no one joining" side of things so I agree with you that post-Wano hasn't done any favors for the "one more" crowd, but I agree with Deicide that Oda will make it crystal clear that the crew is complete. Whether Jinbei is the last one or there is one more, Oda will put something in the story to tell readers that it's over. We still have Sphinx, which should be a powerful, personal arc for the crew and we have Elbaf, possibly the last typical One Piece arc left before the war. There are still opportunities for Oda to let us know that the crew is done.
@last-exit-to-laughtale
I agree that Egghead is pointing the SHs towards Elbaf, but knowing that the Ohara Research is there I think Oda is going to make us wait a bit longer. I think this arc is all about building up the WG and the next arc will be focused on Blackbeard. I think there are more pressing matters that need the SHs' attention first and Oda is going to tease us a bit longer.
As for Wano, I never read that theory before so thank you for sharing that. This is why I'm thinking a return trip to Wano will happen:
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We were told for years the importance of the borders opening and that didn't happen. Oda told/showed us that Zunesha would open the borders. Now Oda loves his surprises and for him to show his hand and not play it suggests something else will open the border and not Zunesha.
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As long as I remember, people have been speculating that three separate factions will each have an Ancient Weapon for the end. Luffy has Poseidon, we believe Im has Uranus, so that leaves someone that would be a rival to both the SHs and the WG to get Pluton. I think it's Teach, though not everyone agrees with me. Right now we do know that Luffy doesn't want it and Caribou knows about it, who works for someone. I don't expect Luffy and his allies to have access to more than Shirahoshi's power during the war.
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Building off of above, Wano is SH territory and the story has been telling us for years what it means to attack an Emperor's territory. Even recently we had Shanks going after Barto for causing trouble in his territory. If anyone attacks Wano for Pluton, the SHs have to come back and deal with the situation. Otherwise, displaying an Emperor's flag doesn't really have meaning anymore, or at least it has a different meaning when it comes to the SHs.
The rest is all Egghead related. This post is getting long so I'll save that for another day. Right now Teach is traveling with Burgess, Doc Q, and Auger and he is one transponder snail away from finding out there is a rock wall between him and an Ancient Weapon. I could very well be wrong about this but I don't think it's a big leap.