Back to the topic, in your opinion, what makes a character "qualified" to join post-Wano especially if it is a newly introduced character? This is assuming there would be another one...
The core principles:
- Be alive
- Be relevant to the story
- Be unique and or have a unique ability
- Be free from faction duty or independant
- Be a friend or a false adversary
- Looking and acting toward adventure
- Having a moral pillar (ex: Pedro / Belmer / Hiluluk)
That's for the core principles, then you get the more specific ones, those are things that all the strawhats are sharing. The character must have:
- Some sort of quirk even if subtle (to be funny)
- A fighting skill (to survive)
- An antagonistic introduction (it's not a necessity, but almost all the straw had one, the character must be a challenge)
- Multiple level of characterization. (it's important for the character to pass through multiple challenge and hardship as well as funny moment and quirky instants)
- A good level of symbolism behind the characterization (it's important to add depth to the character)
- A strong character arc (the character must be put in front of a strong challenge that will make them make a choice)
- Something that will make us think that the guy/girl will be recruted (can be a strong moment with Luffy, or the way the protagonist is written)
- What I call 3 driving force (Basically, it's one desire (a goal, like wanting to be the best at something), a primal need (this is what will make the character look like a human being (for example it is to be able to go through a grieving process), a the secondary need : this is the need to fight for Luffy, this is primordial as every strawhat must fight for themself AND for Luffy) - This is what negate the need for a backstory as it can be developped in the present.
- A post (One of the most important part of being a strawhat. Without a strong post, there is no place on the Sunny)
- What I call the Shining Nakama action (this is once the character gets to the post, it's the shining action of the character in that post, in other word : it is the moment that ciment the character in that post)
- Finally there is the Rescue. (The character must be saved by Luffy from someone or something)
With that you can easily spot the character with the most potential.