I don't think Fishman Island will be in or near the All Blue. Because its at the BOTTOM of the Red Line. I recall someone saying its 20,000 meters down. While theoretically the waters of all the oceans might gather there, it wouldn't be the All Blue simply because its so far down you can't get the diversity of sea life Sanji is looking for. All Blue is likely to be some area of the New World where the seas gather, and species who live in the most shallow beaches to the deepest depths of the oceans can all live.
I'm thinking the all blue will be between raftel and the red line, somehow it'll be explained that some of the water being shot up reverse mountain somehow gets pushed DOWN back the wrong way, the bit under the top current and there'll be like a hole underwater at the very peak so the ships get pushed by the top current but the fish get sucked down the hole, the hole then spits them out the New World side. Idk how but it'll be something along those lines.
@Rubber:
Coby will capture Alvida (Its pretty obvious) and his blonde friend will capture his father Morgan. Assuming Morgan is still alive.
Morgan is confirmed to still be alive he was seen most recently as a cameo in Jango's Mini-arc taking a nap in the ocean as Jango unknowingly passes him, nothing suggests he's dead or in prison as of yet.