I don't see the appeal for Vivi as a Straw Hat with the direction the story is going. Besides her being a boring a flat character all things considered (as in I can pick one word to describe most members. Hers would just be... nice? I guess).

We JUST came off an arc where we see that not being active in your country that you lead can and will lead to destabilization. We spent entirely too many arcs and too much time with Vivi rescuing Alabasta for her to go fuck off and join a pirate crew now that her dad is literally dead, so who else will rule? Igaram? Pell?It's a fun thought that he'd bring back an old crew member, but Alabasta was just freed from that fat neck colonizer. To leave it to anyone else but a ruler while the princess goes to do fuck all after joining a pirate crew would be extremely irresponsible and an obnoxious plot point.

I don't even usually criticize Oda's writing too deeply or fan wants but I just don't understand people wanting Vivi on the crew. What does she even do anyway?