Ok so I know this may seem like a crazy theory but what if Reiju is the next straw hat? In today's chapter, we got to see a lot of her personality and the flashback of what really happened to their mother.
One thing that was interesting to me is how they only pointed out that she still has some humanity left in her in the beginning of the flashback. Although the flashback was mainly about Sanji it left me with the impression that she would be a good fit for the SH's. She checks out with having a broken past and having powers; I wouldn't mind if Oda changed up the recruitment formula to still show he's versatile as a story creator. The only things not confirmed in the SH recruitment checklist is that we haven't really seen her in an actual fight, even though as a child she was shown to perform with superhuman strength, and recently she was captured by Pudding so she is appearing to be on the weak side. Also, we haven't seen if she has a goal or any aspiration outside of Germa66. Like I said earlier it would interesting if Oda made it that she doesn't have any dreams. Or he could present Reiju with a newly found dream, something like to join Luffy and protect people from evils like the Germa. It's so many options and ways to take Reiju's character.
I think the brother-sister duo of her and Sanji would make the SH interactions with each other would change because of Reiju joining. In ways such as how Sanji interactions with nami and robin, do you think it would change? The fight sequences would be interesting as well I'd like to see how reiju and sanji respond to each other while fighting opponents.