I think in this arc, they are just gonna get Sanji and dip out. Big Mom is on a different level and they will fight again soon. The only thing wrong I see in this is that Big Mom and Luffy want to have Fishman Island under their flag.
1: Bege takes advantage of the confusion caused by the Sh and uses his plan to take out BM (or at least shift the focus from the SH to Bege himself). Blackbeard could show up but thats a bit random. Basically third party distracts from the SH
2: Big Mom goes on one of her craving rampages and the SH stop her with the combines efforts of the entire crew. Chopper fixes her illness. Luffy will blurt out/scold her for being crazy and taking out her own kids and when she realizes what her kids have been covering up and the medicine provided by Chopper she will be eternally grateful and let them go. side note to this one its that the SH could take out Bege and stop his coup.