A worthy candidate
Before analysing the real potential of a candidate to join the crew, we must check for that potential is every characters in the story. Meaning that we must check more than 1000 character. Of course we can't end up with 500 character on the point system so we need to observe the story and understand the following:
Remeber, the objectif is to predict the future strawhat. So we must observe the strawhats as they were before the recruitment here. When we observe how the strawhats are crafted before they are officially recruited, we can understand the way Oda is creating his strawhat crew. Oda is creating a FAMILY that is heading toward ADVENTURE and each member of that family are special in the way that they all have their own story related to the main thematics of One Piece. So, when we understand that, we can observe that there are 5 GOLDEN RULES that a character must ABSOLUTELY fulfill just to be considered a worthy candidate for a recruitment at the present state of the story. A character must:
Be alive
Be relevant to the story
Be unique and have a unique ability
Be a friend or a false adversary
Have a guide of the will
Now, I will take a closer look to those rules and begin to sort characters.
1. A character must be alive
The future recruit needs to be alive. Does this mean that "ghost" characters like Brook can't join? No, it means that the character must be able to interact with the world of living.
Therefore, characters like Ace or Pedro can't join the crew. Why ? Because they are dead..
Yohohoho!
2. The character must be relevant to the story
Like stated in the introduction, I will only focus on character present in the story at this very moment. This means that the future recruit must:
Have already been presented in the story
Have had some emotional relevancy to the story
Have had a good amount of paneltime/dialogue
Have had at least one interaction with one member of the crew
This means that character like the Snow queen or mr Snowman are not good candidate to join the crew
Those characters were presented early and had at least one interaction with one member of the crew, sadly they had no emotionnal relevancy with the story nor a good amount of panel time. (Plus they are made of snow, so it doesn't help)
3. The character must be unique and have an unique ability
Every strawhat is unique in order to make each character highly memorable. As such, all of them have their own thing. Zoro can fight with three swords, Franky is a cyborg, Chopper is a reindeer with transformations etc.
The ability or design trait can be passive but it must be something that make the character unique when compared to the rest of the crew, something very memorable.
This is why character like Hiyori or Mihawk are not good candidate to join the crew. Hiyori is a simple human with no devil fruit or ability, beside her design, Hiyori is just an average protagonist when compared to the crew the same can also be said for Mihawk. Yes the man has a unique design, but that's just it. His fighting skills are nothing unique. Mihawk is very close to what Zoro is. We already have a swordsman, for Mihawk to be considered unique he should be bringing some unique ability like the soul soul fruit of Brook or the three sword style of Zoro.
4. The character must be a friend or a false adversary
The crew is the representation of a family, or at least, this is Luffy's one. Because of that, each strawhats are and need to be relatable. Luffy can't accept someone he can't trust or can't forgive. That's why the future recruit must be at least a false adversary on the verge of betraying the ennemy and must have never done something unforgivable.
Note and its important that we must have EVIDENCES of a character being on the verge of betraying the ennemy to be considered a false adversary.
That is also why characters like Ceasar or Enel are definitely not good candidate to join the crew. Here are some argument people tends to make to defend the chances of those two:
"Oda said that an ennemy will probably join the crew". At jump festa 2009, Oda supposedly made this statement : "They will make a former boss a comrade and head to the new world". Well, the story was heading toward Impel Down at the time so it safe to say that Oda was most likely talking about Jinbe.
"Redemption can always happen" Hm, yes it can. But there are rules to make a good redemption arc arc work in a story:
A. The importance of the redemption must be relative to the narrative gravity of the sins. For example, in Dragon ball Z, death is just a path that can be cross with the help of the Dragon Ball. That's why it's okay if Vegeta or Picollo were maniac people in the past. Their deeds can be undone and so, can be at least in the story forgiven. The same can not be said for One Piece, where death carries a lot of weight. So murdering psychopath can't have the same redemption arc.
But there is another parameter that can be juxtaposed to death: Suffering. In some cases, suffering can be seen as something even more serious than death. Suffering makes people cry, suffering - in One Piece - can be traumatizing. That is why a character who willingly made a lot of people suffer can't have a complete redemption arc
B. There is another rule in storytelling, for a redemption arc to happen, it must have been shadowed first! That is why people like Franky and Robin weren't portrayed as complete antagonist. For a good redemption arc to happen, we needed to have at least doubts about them.
The third argument often come with the mention of Crocodile. People tends to think that because Crocodile allowed himself to follow Luffy for a while, an antagonist could follow Luffy on the Sunny. but there is a catch to this argument, it doesn't take into consideration the fact that the arrangement between the two protagonist was nothing but alliance, never friendship and it ignores the most important thing : Luffy never trusted Crocodile. Why should he ? The man made Vivi and his friends suffer.
So, after all that, there is but one conclusion : Ceasar and Enel will never be forgiven by Luffy. After all, both made his friends and allies suffer to an unforgivable point. And all of this without the fact that adding to the crew a genocidal maniac and slave owner or a guy who abused children by giving them drugs.. would be an horrible stand from Oda as a writter.
5. The character must have a guide of the will.
This is where the theme of the inherited will comes into action. This is one of the most important criteria. The message of One Piece is not only that you can push yourself forward but also that you can push forward because people push your back first and helped you make the first steps.
Therefore, each recruit must have someone, a guide, dead or alive, that they can rely on when they are in difficulty. It can be a parent or a mentor but there must be someone important enough in their mind to help them keep pushing forward and make them accomplish their goal.
For that reason Caribou is not a good candidate to join the crew. Caribou has a brother, but there is no evidences in the story that Coribou is seen by his brother as a guide of the will.
Again, we must have EVIDENCES.
Let's conclude
There are 5 GOLDEN RULES that a character must ABSOLUTELY fulfill just to be considered a worthy candidate for a recruitment at the present state of the story. A character must:
Be alive
Be relevant to the story
Be unique and have a unique ability
Be a friend or a false adversary
Have a guide of the will
Now, the question is, who are the very best candidates and why ?