I really hope Fishman Island isn't the mega epic 2 years long Water 7-like saga everybody thought it'd be, and that Jinbei will be nowhere to be seen.
My preference would be for Fishman Island to be a low fighting miniarc introduction to a New World mega arc. There's a good possibility that the WG will tell the shichibukai to not engage the Strawhats after Moria's downfall to avoid further imbalance between the world powers. If luffy's ends up taking down one of the Emperors in the near future, then the WG would simply switch their policy to one of containment and Luffy would then be considered the newest Yonkou.
Also, Jimbei isn't necessarily an evil guy. Sure he was a blood thirsty pirate back in the day, but he may have followed a similar path as Zeff towards redemption. This would explain why he got rid of Arlong and started working for the WG.
So, my prediction is that Fishman Island will be a stopover introduction like Loguetown, where the stage is set for the next grand arc. Jimbei could even aid them in their endeavor to cross the redline if Luffy either helps him out with a problem or simply pays him off (after all, they are rich right now.) Then, they'll find a way to the new world and hilarity ensues.
@ADHD- They are still in the Grandline as they haven't come across any doldrums or sea monsters, and thus haven't crossed Calm Belt. There is still wind and clouds in the sky, so they are not in the calm belt either. Sorry.