Are there any positions on a ship which aren't covered by current members of the crew, this could hint at another crew member.
But so far I'd say the crew is completed and the question is do we get another Momo and Kinemon character who travels with the crew. Wouldn't include Law in that category as he had his own crew and a place to return to.
But Bonney and Vivi could be candidates for that role, but I think Bonney and Vivi are better suited to join the ranks of the Revolutionaries, Bonney as a replacement of Kuma and Vivi as a representative of one of the 20 kingdoms which opposed the ancient kingdom at some point.
I like Bonney's chemistry with Luffy and would love for her to tag along for a while, but her story seems to revolve around Kuma a lot, so what happens with her once Saturn is dealt with, assuming he is finished after Egghead.
Vivi is a wild card, I think Oda considers her part of the crew, along the lines of Momo, Kinemon (and Yamato?). I would love to see her again with the crew! At the start of the Revery arc, Oda made sure to highlight that she gotten stronger, when she jumped down the mast, I think Igraham said something like that? With the revelation that she is another D maybe its her reason for going to Laughtale as well, but so far I think she doesn't know any of that, so first she would need to meet up with Sabo, or its not as important as I think it is and she can join up with the crew regardless of that.
Any Vegapunk could be cool as well, but with the knowledge any of them have might be too much and sideline Robin.
This could be addressed by a memory loss of a satellite or Vegapunk if Punk Records gets destroyed. So essential, Vegapunk tags along with the StrawHats to recover his lost knowledge or something like that.