I'm jumping back on board with the Vivi gang at this point. Seems due to King Im's interest in her, as well as what happened with Cobra's assassination, she's going to play a much larger role in the plot than we thought.
As for the whole idea that she can't be a pirate since she's a princess, in light of what's happened, I could easily see someone working for King Im seizing control of Alabasta in the wake of Cobra dying and his only heir being AWOL. Or Vivi could even have had her status as royalty revoked if we take the Elder's word of 'erasing someone from history'.
Maybe after all the corruption she saw at Levely with the Celestial Dragons and now this assassination, Vivi realizes trying to be diplomatic about things isn't going to cut it anymore, and she needs to actively fight for peace, and decides joining back up with Luffy and the gang will help her do just that.
As for her strength, as some have pointed out, we don't know her current power level at the moment, for all we know she may have been training these past two years in case of something just like this happening.
I could even see her getting a devil fruit power at some point. Personally I'd prefer to see a logia type power for the sake of ability variety among the SHs. Logia is still very useful even if Haki can potentially counter it.
@Shiebs said in Next Straw Hat Crewmates (Vol. 9 - Yamato vs. The World):
What logia fruits are even left? I can’t think of any forces of nature that haven’t been done
I’d say rocks but we already have Pica which is too close in concept I think
I don't know about this for Vivi necessarily, but I do have an idea for a steam-based logia power.
The user could manipulate the temperature of the steam to be scalding hot to burn enemies and even dehydrate them. With enough skill, it could be used to freely control steam-based engines as well.