Honestly with every passing chapter I feel like the most likely and feaseable place the last Nakama could join would be Reverie and that means Vivi. I don't know why but after revealing that her lineage traces back to the Celestials , it's almost as if the crew isn't complete without her.
Because the way I see it after Jinbe joins you have three key characters who descend from races, bloodlines or countries that could expose the World Government apart of the crew. Luffy being a D , Robin being Oharan and now Jinbe the idealistic Fishman . Seems like all they're missing is a good hearted Celestial Dragon to complete the set. It's kinda weird how much Vivi and Doflamingo had in common looking back. Both used strings as weapons , descended from Families who either refused or rebelled later on against the Celestial dragons, both very territorial about their ancestral home.
Only thing she lacks is a fulltime Pirate life and a devil fruit, which is where the fan fiction esque theories come into play. What if Vivi has eaten a devil fruit between the timeskip? , a Peakcock Zoan to be exact, just to keep the symmetry going with the strongest Warriors in Alabasta being zoan types . Maybe that's what that graceful and daring flip from the crows nest was about because I don't remember her being that acrobatic or sturdy for that matter.
But mainly I think the x mark promise is something that's heavier to the narrative than we probably think, we still don't even know if they'll crash this party but could Vivi really see them again and decide not to go along?