honestly I can't see Jinbei joining at all… or even Hancock for that matter, why you ask? Because they are freaking shichibukai they are really strong.
If Jinbei or Hancock joins the crew it'll be pretty much a given that they will conquer shit.
With Jinbei he can destroy an army of ships with ease. Can Luffy do that? Sure he can, maybe.
But could he do it as quickly and as effortlessly as Jinbei? I highly doubt that at this point.
Hence your putting someone who has already established himself in the era which would be trump card for making Luffy the pirate king. He's simply to strong and to famous to join the crew.If Hancock joins you know all those pirates running at you with swords and shit? Send Hancock! and she'll turn them all to stone, replacing the need to see the whole crew in action. Which is stupid.... not to mention her powerful developed Haki powers which is again a trump card.
But eh, this is my opinion :)
I agree with Jinbei.
And I don't see Hancock joining either. But your point about sending Hancock against the fodder…what if most of the New World 'fodder' is actually strong enough to negate her DF ,Momonga style. I know it's a big 'what if' but still.
I do think Hancock will travel to the NW looking for Luffy, because i don't see her wanting to leave Luffy except if Luffy does something she detests.