@ShadowOfTheSword:
1. I know that. A guy can hope though, can't he?
Base theories off the story and characterization and plot, not random whim.
2. Rebecca would be interesting to join because she is a swordswoman. And she's one of only two I can think of.
Describe her personality.
And if you can't think of any swordswomen beside Kuina and Tashigi you aren't trying very hard. Mozu, Kiwi, Lola, Aphelandra, and Baby 5 off the top of my head. Whitey Bay, Haruta and Elmy as background-ish characters with minimal screen time. Along with about 100 male swordsman. Including two of them on the crew already. (And one of Franky's mechs has a sword as well, so…)
3. Jinbe is boring because his past really wasn't that tragic.
Death of his mentor, betrayal of his comrade Arlong, war against the humans, seen the affects of slavery, guilt of what he allowed to happen to Nami, pressures of protecting the entire kingdom, betrayal by Hody, friend of whitebeard, former warlord who has gone against the government, has strong ties to Luffy (and was there in his darkest hour), a pledge of promise to the now deceased Ace, the respect of the entire crew…
and we haven't even gotten his PERSONAL flashback yet, we've only gotten the Fishman Island one. His personal stuff will come along when we find out how he got the scar.
Rebecca, on the other hand, has the potential for a tragic past.
Jinbe already has that. And was accepted into the crew.
And Rebecca has a grudge. Against Doflamingo.
So does Bellamy and Momo.
Besides, the Straw Hat Pirates would have to get past Big Mom and her crew to get Jinbe. And there's no Law to help Luffy take Big Mom down. The way I see it, that alliance is going to be dissolved when (if) they beat Kaido. Is it really worth taking a Yonko down alone to get someone weaker than you on your crew? I hope Luffy realizes that.
Is it really worth fighting the highest bounty in East Blue Arlong just because he made Nami cry?
Is it really worth fighting a Warlord like Crocodile just to help Vivi out?
Is it really worth going against the entire world government just to save Robin?
is it really worth fighting a warlord like Moria just to help Brook, who Luffy had known for about 8 seconds at the time?
Is it really worth summoning the wrath of an admiral so Luffy could punch an official in the face to help out Hacchan?
Is it really worth recruiting Usopp in all his cowardice and weakness? And sure Chopper is a doctor, but he's not very strong either. Nami? Forget about it.
Luffy is angry with Big Mom because she threatened Fishman Island. And because he's going to be pirate king. Not because of anything really personal… yet.