The only problem is, if they go forward, it's the Red Line again. They HAVE to pass MArijoi if they want to go any further on the Grand Line. Backtracking is optional, but it's not something they would exactly want to do.
As for Jimbei, I see him as kind of the emperor of Gyojin Island, a hero that lords over the most powerful of the Gyojin.
Here's what I see for the Jimbei Arc:
1. Jimbei ate the Mizu Mizu no Mi. His moves are stuff like "20,000 pound pressure geyser" and "Surface Impact" (He makes a wall of water and uses a geyser to slam his opponent into it, creating impact).
2. Gyojin Island is an Atlantis type city, with an "upper" city on land with Jimbei's palace, and a "lower" city underwater inside a dome.
3. One of the three admirals will show up during this arc. Most likely to observe, but it's possible Kuzan could defeat Jimbei (By freezing him).
4. Nami will be the one to defeat Jimbei if Kuzan doesn't, either by evaporating him, or electrocuting him (Cloud Tempo or Lightning Tempo). Either way, Jimbei will recognize his weakness to Nami's climatact and take the battle to the underwater city.
5. Zoro's opponent will be a Crab fishman who uses 5-sword style (4 claws, mandible/mouth). He'll also have an extremely hard shell, so Zoro has to learn a new cutting technique to defeat him. Zoro will take part of his shell as a prize, and it'll be used repair Yubashiri.
6. Sanji and Franky will fight a Seahorse Fishman.
That's it for the predicitions.