Will either Franky or Paulie really be joining the crew after the Water 7 arc is over? For some reason I get the feeling that Oda is throwing us a curveball by introducing us to Franky and Paulie as potential carpenters for the Strawhate pirates crew (like how he threw us a curveball by introducing vivi as a potential new mate). Franky has his own crew already, and his dream is to build the ultimate ship. So there's no reason for him to join Luffy's crew once he builds the ship. Plus, if he's anything like his mentor Tom, once he builds his dream ship he'll be staying behind. As for Paulie, he just doesn't fit with the crew. IMO, his character just isn't colorful or unique enough to be a new crew member. Plus he's sort of weak. So if neither Paulie nor Franky will be joining, who's going to be the future carpenter?
Here's my theory, which came to me after revisiting the Skypeia arc (even after telling myself not to and wait for Kaizoku subs, couldn't help but watch those evil evil HK-subs - needed my OP fix ^_^). Recall the scene where, while the crew members are on land, and the ship still on the altar, we witness a mysterious ghost-like figure repairing the ship. We never did find out who that was after the arc was over. Interestingly enough though, the head of this mysterious figure had an uncanny resemblance to the figure head of going merry. Which dawned on me. What if that unknown figure was actually the ship itself personified? If the Luffy crew should ever meet this figure again in the future, IMO, he/she/it will probably be the new carpenter. This would actually make sense, because it would fit with one of the story's main themes - that is friendship and loyalty. Everyone in the crew regarded the ship as a nakama. I thought it unbelievable that the crew could just abandon the ship like that in W7, because it poked huge holes in the friendship theme the story was trying to convey. The crew (other than for Usopp) seemed to have shown no real loyalty to the ship. Yes, the ship is now destroyed, but as we all know, no one, or no thing really important actually dies in One Piece (since Oda wills it). In which case that, and the theme of friendship and loyalty (which collapsed when the ship was abandoned), can be resolved quite nicely if the Going Merry itself is introduced as a real walking talking entity and joins the crew as the new carpenter. My gut feeling tells me, this, or something like it, is what Oda has in mind for the story.