Well so far, they have no Logia type party member, granted they don't NEED one, but so far the group is pretty diverse that a logia is the only one they can have that doesn't duplicate another party members, we have Luffy with his Gum Gum fruit powers, Zoro with his sword skills, Usopp and his ranged/pop green attacks, Nami with her Climate based attacks, Sanji with his kicks, Chopper with his forms, Franky with his body(god regret typing that)Robin with her devil fruit, and finally Brook with his "undead" powers and fencing, Logically we don't need another swordswomen, but another "ranged" type party member would even out the front line and add another women to the group, She does have some things going for her, a flash back, this early before we even meet her really, her father is the king so that makes her a princess, which the group currently does not have any royakty, and her name could be Rebbeca the Buring Princess, or something along those lines, and I doubt that Oda would add another character like Vivi and also keep her behind to stabalize her kingdom. But of course we will see, there is know way to know what Oda is thinking from one moment to the next.