@Jeff:
Sure, why not? Is it inconceivable that another character will appear at a later junction, with a different back story worthy of its own flashback? It's probably not going to happen during FI though.
You just asked why it wouldn't happen, and then in the same paragraph you say it probably won't happen…
Having some new character come by with an extra flashback will have a very difficult time following a flashback that's been foreshadowed for over a decade now. We're entering THE mega important flashback of the entire arc. This is the time.
I'm more confident about an introduced character than Jinbe because I see no concrete reason, as of yet, for Jinbe to join.
What is "concrete" exactly? Absolute proof? Nobody would have such a thing. That's part of the reason it's interesting.
Sure it narrows things down, but in a world where there are thousands of red sports cars you're can't just start assuming it's the first one you come across.
No, but the fact that it fits the criteria means it's worth investigating until it doesn't.
Timing is essential for introducing a new crew member. The fact that Jinbe was introduced all the way back at ID and has been a fairly well explained character, sans a flashback, already makes him a different beast than everyone else.
Every time a new crew member has appeared there has been something about them that seals the deal pretty early. Zoro was the first one Luffy met and Luffy went in with the attitude that Zoro was going to Join. Nami was a navigator and Luffy was hell bent on her joining. Ussop was the son of Shank's crew member and a personality that Luffy enjoyed from the start. Sanji was a cook. Chopper was a doctor. Robin was a strange case, but she was a character who we knew little about when she joined and it was clear that the Poneglyph thread was central to the plot. Franky was a shipwright and as soon as he started buddying up with Ussop it was pretty clear what was going on. Finally Brooke was asked to join right when Luffy met him, pretty clear.
He was first mentioned while Luffy was still in East Blue. Also, Oda tends to enjoy not rehashing the same thing over and over. The fact that a previously established character hasn't joined yet, to me, means that it's about due to happen.
Jinbe has nothing that makes him a clear choice. He was introduced a long time ago and he has only differentiated himself from the pack as a mentor/support role, similar to Ivankov and Hancock. Oda doesn't pussyfoot around when it comes to new crew members. If Jinbe was going to join there would be a far more clear indication as to why before he gets any sort of flashback.
Ah, I see, Jinbe supported Luffy. Luffy's crew is different… somehow.
Tell me why, in the context of that statement, Franky was different.
Of course every flashback is involved in world building, but the scope of past flashbacks doesn't even come close to this one. The actions and deaths of both Tiger and Otohime explicitly involve far more than just Jinbe. Past flashbacks have not been on such a grand scale. Specific crew member flashbacks have dealt with events that shape single characters, this one is about to deal with events that have shaped the entire world.
I know, this flashback is set up to be absolutely incredible. To not get a new crew member out of it would be a real shame.
How about the entire country? Notice when the NFP go around asking the citizens to stomp on Otohime's face that there is hesitation and resistance. Clearly she was a figure of respect and I'm positive that most of the country agreed with her views. Same thing goes for Fisher as well. Just look at the NFP, they are the youth that are taking his viewpoint way too far. Jinbe has presented to us as a character who initially believed in what Tiger had to say, but was conflicted and eventually agreed with Otohime. He is acting as a proxy for the entire country because surely there was a divide between who the country thought was doing it right.
Ok, I'm pretty sure proxy isn't the word you're looking for. That would mean that everyone goes to him for help. Maybe something like "symbol" would be more appropriate. Anyway, getting away from word definitions, I agree. Jinbe can understand both sides. That just… improves his chances as far as I can tell.