Like i said before, i dont think Kinemon is going to join, it just doesnt feel like it to me.But why do people keep saying that Kinemon cant join because he has a son or because he is a swordsman?
Having three swordsman is neither bad nor abnormal for a pirate crew.Swords are the main and most badass weapons on one piece world.I am sure Oda could make the crew work with three swordsman.Also they are not exactly similar.Brook is a swordsman but he has a lot of other feats, and his swordsmanship is related with cold.
Zoro is not like Brook, he is defined by his swordsmanship.His swords are everything to him.His swordsmanship is quite unique having three swords.After timeskip he seems to be more focused on ranged slashes like Mihawk.
We dont know a lot about Kinemon but he has a DF, so if he joins he will have feats other than his swordsmanship like Brook.His swordsmanship is mainly related with fire.
As you can see they are not too similar.They have their own personalities and feats.
By the way on a side note, speaking of Brook, i really want to see him singing "Bing's Sake".ITs been a while, so i hope they make a celebration soon
Also we dont know anything about his son.It might turn out that his son is actually a thirty-year-old man who doesnt need his father.It might turn out he is dead.He might leave him back at home.There are a lot of possibilities.
He might already have a dream.
We know almost nothing about him, and we cant say he cant join.We have to learn more about his background, about his son, about samurais in general before we can accurately speculate about why he can or cant join the crew.