I don't think Franky will be the 8th crewmember. Something about him seems off. The way he looks, his voice, and even his back story doesn't seem to fit the rest of the crew. Yeah he has a tragic past, but it doesn't seem so tragic compared with everyone else. The things that happened to him was his fault, whereas the rest of the crew are victims of the situation. He shouldn't have built the warship, but at the same time, Iceburg has been telling him that for years. Also, he has Franky Family. Where ever he goes, they'll go with him. I think the 8th crewmember will be someone not shown. Someone we haven't met.
Here's my take on what might happen. Goin Merry sank. Even if you bring it up, you can't restore it. However, you can salvage pieces of it. The head piece is probably the most important. They're going to put that on a new ship. They may even use the same wood from Goin Merry to built parts of the ship. This way, she will still be part of the crew. They can portray it as her being reborn or upgraded. While everyone is fighting in the Lobby, the 8th crewmember will be working on the ship. It would probably be his/her design. Then, when the Strawhat pirates come, he/she will surprise them with the ship and ask to join the crew. We'll then know more about the person as more episodes go by. Oh, and the reason why Luffy lets him/her into the crew so easily is because as a hobbie, he/she is a musician.
