@Sparsebeard:
A friend of a friend tried to convince me that the barcodes on products was a satanic ploy to put the "number of the beast" everywhere lol.
You mean it isn't?!
Just joking. But seriously, I don't think there's any sort of pattern you can identify in terms of people joining the crew, it just wouldn't make any sense. Instead, you have to examine it from a story perspective. Each character's joining added something to the crew, whether it was in-story or for the purposes of lore. Chopper was a doctor that they needed, but it allows Oda to have a childlike naive character to use as a vehicle. Robin diversified the crew by adding another female, but from a story perspective she's the only one who's primarily connected to the lore of the story. The biggest mystery, surrounding the void century and poneglyphs. She was also a good way to show how far the Straw Hats would go to save one of their own, and she's one of the most intelligent members of the crew. Nami may specialize in navigating and weather science, but Robin's got worldly knowledge.
Franky was the shipwright they badly needed, and was also the first instance the story had of technology and how advanced it really could be in this universe. When Pacifistas show up, they aren't that hard of a concept to swallow because we've already seen Franky rebuild himself from scratch using derelict ship parts. If it wasn't for Franky, things like lasers, robots, and other stuff in the story might seem out of place and weird. Instead, it was built up to those things which is why it doesn't. The first half of the story was fairly restrained in terms of all of this. Finally, Brook doesn't have too overt a connection like that, but he's the musician Luffy's always wanted. This doesn't come up much in the story, but he was a pirate even before the great age of pirates, so he does have experience from that era which could come into play later on.
Personality wise he's able to join in on Chopper, Usopp, and Luffy's antics, as well as Sanji's perversion. As a character, he rounds out the crew very well, which is fitting as he's the musician intended to entertain them all, and keep their spirits high.
Now, as far as some of the "points" that were mentioned by ktost, the problem is that most of these can be retroactive in hindsight. See, dreams, sad pasts…even fighting ability or strength, these all only serve as vehicles in the story. They're all storytelling devices. Sanji's fighting style is perfectly based off of his character. Chopper's newfound confidence in combat after the Time Skip is evidence as to how much he's grown up as a person. Before, he only won against Gedatsu cause Gedatsu's inept, and had to resort to Monster to take out Kumadori. There's no pattern here. Every character's strength is determined by that character alone, like how Brook is relatively weak cause he spent 50 years going insane on a ship. He didn't spend that time training. It also makes sense why he was relatively unfamiliar with the possibilities of his fruit up until post TS, he never had a reason to really experiment with the DF before.
Also, I'm not Japanese, but I think we've all read enough of One Piece to know that Oda loves his puns, and Japanese as a language can facilitate that very easily. One of Doffy's attacks is named Obahito, (Overheat). The Ito part translates to String. But there's nothing inherently fire based about that attack, it's just a pun. We've seen it constantly. The numbers thing that was mentioned, about how if you add up all the numbers it becomes either Smoker or Kinemon, is nothing more than a coincidence. Japanese seems to constantly have things that can be read either way depending on context, and how it's read.
As far as the male/female/male/male pattern goes, this one's shot to hell already. You have to argue whether or not Nami actually joined after Zoro or not, or whether she really actually joined after Arlong. Similarly, did Robin join at the end of Alabasta, or after Enies Lobby? She was only with them for the entirety of Jaya and Skypeia. After that Water 7 started. Kinemon's been in the crew for that long too, so is he already part of the crew or not? (Answer is no, of course).
Finally, the unique fighting style and relevance to the crew is a clear indicator of someone joining the crew. But as far as the favorite food, favorite color, nation they represent, animal, and all of that shit, it's not stuff you think about when designing a character, it's stuff that comes later. When Oda had to answer in an SBS what everyone's favorite food was, it probably wasn't that hard to figure out. You take one look at Franky and could believe he'd like stuff that's associated with cola, like burgers and fries. Anyway tl;dr there's no pattern, and if you honestly think there is you're not giving Oda enough credit, and seriously undermining all the little things that make up this story. One Piece is absolutely dripping with characterization, elements, and aspects that you may not even realize the first time reading it. It undermines all that to say that characters will join based on some arbitrary, shitty pattern.
it might as well be that Monet picked up her heart herself btw, then proceeded to put Vergo's body together (hopefully with some parts in the wrong place withtout him noticing 
