During the WB arc, WB seems to be winning and getting the upperhand. Victory almost seems there, as Ace is rescued, and they are ready to GTFO. However, then BB shows up and shakes the foundations of the place, and ends up killing WB. Ace, and everyone is in shock, but they decide to hightail it, as WB would've wanted them to continue living and not die trying to avenge them there.
Word of WB's death causes chaos to ensure in the world. Fishman island is thrown into dissaray, as WB's hold over it is gone. Fighting among the three remaining Yonkou erupts to take part of WB's territory, while the WB try and recover from their drastic loss. Meanwhile, Luffy returns Shaobody, where his crew slowly return one by one until they are all back. They recount their individual adventures and their individual power-ups or epiphanies before finally embarking onto fishman island.
On Fishman island, instead of the glorious world they expect, it is instead in chaos. Luffy and co. solve the problem, defeat the main antagonists, and restore peace upon the place. Luffy's fame is unparalleled with his incidents at Impel Down and Marine HQ at this point, so he declares (similar to WB), that this land is not to be trifled with, unless they wish to trifle with the SH pirates.
Luffy and Co. would then also have an arc involving slave-trading and Donflamingo. This could even be wrapped into Fishman island, as fishmen/mermaids are the best slaves (money wise).
The next arc would involve Vegapunk, Sentomaru, and Kuma. SH would get tons of revelations here, Franky would also get some growth, and they would also get their revenge on the three from before.
We'd also have some arcs, involving the other Supernova's, possibly mixed in with the other arcs I mentioned above. There would be arcs involving both Kaido and the last unknown yonkou. Lola's mother would also play a role in helping defeat one of these foes. Then there would be an arc where Luffy and Shanks finally meet, and possibly ome revelations there. Arc to settle the rivalry between Kid and Trafalgar. Then another for Mihawk and Zoro (mixed in with a bigger arc). Finally, it'd slimax with the BB pirates and SH for Raftel and eventual rights to be Pirate King.
Kind of fuzzy, but those would be the major points I'd have to address