I've seen scattered discussion regarding this in various topics on this board, so I figured it would be best to consolidate the theories / opinions here.
We know that Oda very rarely does anything ad hoc; there is always a purpose that is eventually revealed to us. In this whole Amazon Lily / Impel Down arc, we've only had one reference to the rest of the Strawhat Crew, at the end of Chapter 523 and the beginning of Chapter 524. This reference told us the location of each of the members and gave a tiny bit of information about each location.
So my question is, why did Oda send them to these locations? Answering that, how will they accomplish whatever the answer to the why question is?
I also think that it is EXTRAORDINARILY significant that Kuma put them there (I'm sure he knew where he was sending each of them, which implies that Kuma is an ally of the strawhats, which, with his connection to Dragon, is pretty damn cool).
Here are my thoughts on why each character was thrown where he / she was.
Nami - Sky Island "Weatheria" - I think this is obvious. Nami's battle prowess and ability as a navigator all depend on her ability to predict, manipulate and understand the weather. I think she'll simply have to study with the people of this island to improve both as a navigator and in battle.
Franky - Future Kingdom "Barjimoa" - Obvious again. The island is a future kingdom, the land of the genius where Frankly will be able to upgrade his arsenal of weapons and increase his knowledge of materials / technology.
Sanji - Island of Dreams "Momoiro Island" - Now we get to the unobvious ones. Well, I don't see how Sanji will improve his battle prowess here, but I'm certainly up for listening to some creative theories, lol. I can see Oda placing Sanji here to maybe grow as a person, to become more accepting of a different kind of beauty…but seeing as how that flaw is part of Sanji's charm, I doubt it.
Ussop - Boin Archipeligo, Swindling Forest "Greenstone" - Again, I have little idea as to how this will help Usopp grow as a character. It seems like the island is an island of trickery however, which is Usopp's forte, so maybe he will gain some advantages in that department. I was hoping for something more related to sniping though.
Brook - "Namakura Island" The land of poverty, "Harahettania" - Not sure what Oda is trying to pull here either. I actually have no theories here.
Robin - East Blue, Bridgetop Country "Tequila Wolf" - This one is a little more telling. A bridge that has been built on for over 700 years. There is obviously some historical significance that Robin will learn here. Important for character development.
Chopper - South Blue, the land where birds rule over men "Torino Kingdom" - I like this one from Oda. Chopper, a beast turned humanoid, prejudiced against his entire life, now consorts and socializes with men who have actually been biased against for their whole history and who are no longer the superior beings. Not sure how this helps Chopper medically or regarding his battling skills, but it definitely is appropriate regarding his character.
Zoro - "Kuraigana Island" the ruins of the Shikearu Kingdom - No idea here. I enjoyed Perona's bit with her battle with Usopp, so I don't mind her coming back. Zoro is a solitary individual who trains by himself, so maybe a dark, dreary venue in the middle of an abandoned ruined kingdom is best for some individual training. I'm curious to see how Oda connects Perona to Zoro's growth.
Basically, I can see why Nami, Franky, Robin and Chopper were sent to their respective locations. I can kind of see why Ussop was sent to his location. I really have no idea for Sanji, Zoro and Brooke though, which is disappointing.
Any way, any theories you guys want to share regarding the purposes of those locations and the growth of the characters sent to them?
I have a feeling that this ID arc, supposedly lasting for a year, won't be only Luffy, and we'll occasionally get allusions to their growth, so I think it's important to see how they might be developing.
To add to this idea, we can try viewing it backwards as well. Instead of location determining progress, you can also state where you would like to see each member grow and how that could be fit into the proper location.
Let me hear some ideas!