heh it is fun to find other people have already pondered and discussed what I've gathered from Oda's writtings. Personally I was thinking the island was going to be "destroyed" by damaging the roots of the Light Tree Eve. Caribou is who I most likely peg as stealing the princess and heading straight up the root system to sell her off in the holy land, which would put the Strawhats in the position to damage the Tree (or the tree damaged by Noah.)
With the battle moving up to Noah, I'm thinking Zeo might catch up with Sanji. Zeo's body appears slippery/evasive and it would be a good chance for Sanji to show off his footwork against an opponent that blunt force usually wouldn't damage.
Hyouzou, the poisonous swordsman, seems the best matched for Zoro, as Zoro needs to become the greatest swordsman by being able to defeat all other swordsmen of great skill. I don't know enough about the other Fishman pirates to judge their abilities versus the Strawhats and which would best show case the powerups.
Vander Decken hasn't really shown much fighting ability besides throwing random things and having a rifle. If Luffy is his opponent I can't see the battle lasting long, even if Decken's Devil Fruit ability can be transferred to his rifle bullets.
Many of the 'evil' villains in one piece have ideological changes after battling the Strawhats, so there is an off chance that before the arc is over that some of the villains could assist the Strawhats.
From my understanding of Decken's ability, the Noah will have to touch Shirahoshi or hit something solid before stopping (the pirates and weapons thrown at Shirahoshi in her tower did not move after she left the room so I assume Decken's power had dispersed.) So far the options I've seen have been she touches it and it sinks to the fishman island (really shouldn't do much damage hitting her in the water,) she summons the giant seakings to stop it, or (my theory) being kidnapped by Caribou, the Noah follows up to the roots of the Light Tree and gets stopped by hitting the roots.
Anyways, the root system of Shabondy was shown to have a lot of Seakings and sea creatures swimming around so that might be a good place to go with the princess's ability, or Water 7 where numerous housing districts already exist under water. No reason why both places couldn't be colonized by fishman island refugees. Perhaps the ones by Shabondy would serve as escorts through the completely changed Fishman island route beneath the Red Line.
Holy Land Mariejois just seems like a large place for the Strawhats to pass by, with all the hype about Fisher Tiger freeing slaves there. In an obvious complete guess, I think Strawhats hit it up trying to save the Princess, meet up with some familiar faces there, and ultimately are saved by the Noah carrying the entire population of Fishman Island.
[[SIZE=1] I have suspicions that Jewelry Bonnie has a back story like Sabo and may be currently forced into being a noble, as well as a legendary carpenter like Tom would be worth more alive as a slave to the world nobles. Additionally Bonnie would be an interesting temporary or permanent crew member, especially concerning the jokes that could be played on Sanji with her ability. Strawhats need more female crew members regardless.
A trip to Mariejois would be a good chance to show off the full new powers of the Strawhats against vice admirals and captains. Also an opportunity for an update of the Strawhats' bounty posters and their nicknames to more approriate forms after the timeskip. (I.E. Chopper upgraded from pet, Sanji getting a real picture, and another guess ~Zoro becoming Demon Eye'd Swordsman after it is revealed he has acquired an eye similar to Mihawk's) ]
The piece of paper that Otohime brought back seemed suspicious coming from world nobles, they don't seem like the compromising type. The Reverie backed by the nobles might grant some sort of protection or immunity, but it could also be a trap to procure slaves. It will be interesting to see if the Strawhats are a part of that event.
I'm looking forward to all of Oda's imagination and hope he continues to surprise us. I love One Piece =)