Although I do think that Rebecca might be a possible choice, since even if she lost Luffy might give her the DF (I am convinced he'd rather die than let anyone untrustworthy have his brother's power), I have a feeling that Luffy will be in a pinch. He will have to decide on a whim what to do with the DF because otherwise it will be out of reach for him. As I said, possible outcomes are...
a) that he gives Rebecca the fruit since she has proven that her intention is to save someone dear to her (just like Ace was to Luffy) and reflects some of what he had gone through in Marineford. Giving her that power would enable her to save the one most important to her.
or...
b) Luffy might eat the fruit himself. I do know that it sounds crazy at first but I do think that it might be possible. There are a few reasons why actually: Luffy does not want to hand over the Mera-Mera-no-Mi to BB, who in turn would either claim the power as his own or give it to one of his crewmates. Thinking about it I would be sure that BB would absorb Ace's power just like Whitebeard's. And that's the thing that's bothering me the most. There are a lot of theories out there stating that BB could take over Whitebeard's power because of his DF. But I have a slightly different take on the matter...
Blackbeard is a man with the middle initial D. and there are only three others with the same initial. What if the power to absorb more than one DF was reserved to the people of the Will of D? It would at least explain a little just why the World Government is so desperate to bring these people under their control or eliminate them if unable to do so. Also, there would be more of a balance in power. BB would otherwise be completely overpowered compared to Luffy or anyone else in the One Piece world. By having someone who can pull off the same trick, he might faced with a serious threat. One bite from a fruit is already enough to get the power.
It would also be kind of a beautiful justice since then Luffy would still have a part of his brother with him.
Also, yes, I know that it was once mentioned that people might explode should they eat a second Devil Fruit but I learned that with Oda you can't believe anything that is said until it's shown... Two times he made me believe that Bon-chan was dead and two times he proved me wrong!!!!!