This is something I have been wondering for a while now. As we know, there can be only one user of a particular DF at a time. In this regard, when ace died it likely resulted in his fruit somehow spawning somewhere else and is up for grabs for whoever finds it. This would be important because this is a power which would have strong emotional value to luffy. In this regard, any development concerning someone using this fruit is likely to have a strong impact in the story. To be honest, I would like to see this fruit being used in one out of two ways plotwise. The first would be that an ally of BB has it. It would make the whole BB/luffy dinamic more interesting, I think it would be interesting to see how luffy would react to meeting a fire user as a member of BB crew. The other would be to see a future crewmember using that fruit. It would perhaps allow luffy for a bit of redemption for him being unable to save ace back in the day.