At this point I really hope the crew closes with Vivi's return.
I've always had a problem with the idea of new crewmates joining in the New World because, after so many years since Brook, a new addition would feel "out of place"; also, we're almost in the ending of the journey. That's why someone like Jinbe was okay to me: not only because I really like the character, but because he was mentioned in East Blue and first bounded with Luffy in Impel Down, being his biggest support after Ace's death. So in a sense he was there since before the timeskip and was linked to Kuma's separation in a way.
Considering what Luffy stated about getting ten crewmates plus how Blackbeard has his ten titanic captains I can only see one more person joining, and Vivi fits. Since she's considered an ex-member of the crew and the Nefertari family was the only founder to not move to Mary Geoise, I'd find it really weird that she didn't end up accompanying Luffy during his last travel. You don't forget a nakama so easily, after all, and while her position as the princess of Arabasta was usually stated as a counter-argument, her current situation may be way different. Considering what may have happened during the Levely and Im's focus on her, an outlaw Vivi is a strong possibility today.
Sure, she's theoretically weak, but this can be solved by Oda easily; for example, a devil fruit could have done the work (even Monet's once she joins! A dessert queen with a snow logia would be interesting, wouldn't it?). In fact, my personal fantasy is that Vivi spent these years travelling under the pretext of "political duties" to islands all around the sea in order to learn from the Straw Hats friends and become stronger in the process in case Luffy needed her in the future. Imagine, for example, that she improved her peacock slashers learning from Paulie's rope action. This would be a creative way to return the help the Straw Hats gave to their friends throughout the journey, since all of them would have turned their last crewmate into a force to be reckoned with.
So that's my bet: Vivi's return and that's it, like a circle closing.