Even if Franky is an ok musician, it's not his speciality. As for saying musical weapons don't have a lot of variety, that's like saying kicks don't have a lot of variety, or swords don't have a lot of variety. It's not a very good argument. Just off the top of my head, there's the hypnosis possibility someone mentioned, and then the resonance effect which could be used to attack specific body parts using different frequencies. And who knows, there could be something like songs acting like spells, and if the song is played through to the end it brings a certain effect to the opponent. Also if they are a master musician, they might be able to use many different types of instruments which could be fun. The thing that's interesting about music as a weapon, is that music would attack from the inside, as opposed to most of the attacks we've seen so far which attack from the outside.
The musical frequency person in this thread was me. Problem is, it was done in a filler and Oda gives the yay or nay on fillers based on whether or not he wants to do something with them in the future. . .
Blow dart clarinets anyone?
I think that they are going to get both Paulie and Franky personally.. If they get a big ship. Remember Foxy's ship? I think it is going to be that size.. And Foxy had plenty of shipwrights (although the circumstances were a bit different) so I think that Paulie and Franky will both join.
I wouldn't be completely surprised if Kaku joined (I would be surprised, but not like when Robin joined) but Rob Lucci joining is.. Rob Lucci doesn't seem like the kind of guy that gets along with other people very well and probably wouldn't fit in a pirate crew. Kaku seems better in that department and probably the best out of all CP9 members
I DOUBT it'll be a Foxy-sized ship, but it'll be bigger then the Going Merry for sure - I personally think it would be cute if Oda designed their individual rooms and we got to see their personalities, or sides of their personalities we didn't know about before. But that's just me. None the less, Foxy's ship was big because he had a crew of 500-something men and growing (plus lugging around all the stuff for DBFs. . .).
But you do raise an excellent point. They might need more than one shipwright if they're going to have a bigger ship. But if Paulie and/or Kaku joins in addition to Franky (assuming Franky joins), we need back stories on him/both of them. So far we have some insight into Paulie's background, as he is a gambler and hates scantily clad women, but we don't know much about Kaku.
Kaku, to me, seems like the "nice" CP9. It's almost like a "well. . . I really don't have anything PERSONAL against you, but I kind of have to kill you now" type thing (or at least that's the expression of Kaku I've gotten). Lucci is just an asshole, IMHO.
My other Kaku opinion is this: a theory I have stated in a few other threads is the possibility of an ex-Marine joining the Straw Hats, which DOES have historical context - English sailors left the British navy because the conditions on pirate ships were actually a lot better. Oda likes to use the names of historial pirates in One Piece and reference pirate lore - adding an ex-Marine would not only give the SHs INSANELY GOOD insight as to how the WG works - strategist, anyone? - but would fit into historical piracy as well. Kaku, a member of CP9, would fit the bill for this kind of job (and double as an assistant shipright).
The only problem would be that Kaku still isn't on great terms with the Straw Hats and his fight with Zoro did get a little nasty (not to mention the events in 420 couldn't have been good for his health). Robin was saved by Luffy AND tried to kill Crocodile - Kaku hasn't done anything of the sort.
Marines who COULD turn to the Straw Hats (using my own personal logic):
Jango: Duh. He was a pirate once. And even though it wasn't canon, he got along great with Luffy in the "Jango's Dance Carnival" short.
Tashigi: Her thing with Zoro is basically one big misunderstanding. If she knew about Kuina, I really, really think she'd respect him a whole lot more. Might not be enough for her to ditch the Marines, but it might make her question her beliefs.