Shanks has already been kind of set up as the ultimate sacrifice to fuel Luffy's battle against Blackbeard, who is, as far as I can tell, the actual final boss of the series.

A good twist would be for Jimbei to not actually be able to join the Strawhats crew and get offed here by big mom, cementing Luffy's declaration of war on her from 100+ episodes ago. I also wouldn't be surprised to hear something like "Big mom was the weakest of the four emperors; thats why she waited for Whitebeard to die before vulturing his claims, and was courting Sanji's family." after Luffy beats her. Also, a hammerhead fishman daughter? Mom sure gets around...