Personal thoughts on how I think Fishman Island arc will play out:
After Luffy and the SHs rejoin, Jinbei will accompany them to a now chaos ridden Fishman Island where Doflamingo, the Celestial Dragons, Marines, and a Yonkou are taking advantage of the chaos. Akainu has recently been promoted to fleet admiral while Sengoku is demoted. This arc could finally reveal Doflamingo's past. The fact he was linked to the auction house could put him back in control of the auction house and use it. Then, Kuma arrives, reveals that he's been in control, and betrays the WG one final time in order to save the SHs, and defeats Doflamingo, but somehow dies in the process. Doflamingo retreats and is removed from the Shichibukai by the WG.
Then, Jinbei and the fishman arm themselves and defeat the Celestial Dragons, and repel the marines for a short time, causing marines to retreat.
Finally, a Yonkou (The one that defeated Moria or the unrevealed one) shows up to claim control of an already devastated fishman Island, manages to beat down Jinbei and his forces, along with every SH, but fails due to Dragon showing up, along with multiple revolutionaries. The revolutionaries free Fishman Island, and Jinbei stays to keep peace with Archipelago, and Hachi joins the SH crew, feeling that everything is as it should be.
In the midst of all this, Blackbeard rips apart Mariejois and Hancock and her people defend themselves, soon succeeding in defeating the marines trying to take them over.
By the end of this, the Shichibukai completely disband by order of WG, as Kuma is dead, Doflamingo, Hancock, Jinbei, and Blackbeard betrayed the government, and Moria and Mihawk are hardly intimidating or powerful by themselves, especially since Moria was already defeated.
Beyond this, the marines become vastly more dangerous and powerful.
Anyone else agree? Personally, I think it'd answer many, many, many questions, and it'd be a great beginning to 6th Saga. I actually do think Hachi will join, he seems like an incredibly appropriate member. Despite being so attached to Camie, several characters have had to leave people they've been attached to, and Hachi could easily have an equal rights dream that could/will be realized by the end of the series.