Seems to be leaning more and more to Rebecca being the Mera Mera no Mi user, as she actually wants it. Bart wants to win it and win Luffy's favor.
The destruction of the Factory will turn all the toys (King Riku loyalists) back to normal and with the King and His son (Next King) still alive to rebuild the country (Rebecca's father, will be next king anyways so no reason for Rebecca to stay like Vivi as she was next in line for the throne)
It looks like, Rebecca is going to be the next Nakama and the first (maybe last?) logia user to join the Strawhat crew. Either way, someone predictable but surprising turn of events. (I figured, Rebeccas father was the Gladiator and the soldier was an old Riku Kingdom old guard/one of the most loyal body guards of the family. Right and wrong, whatever)