My bet: Sanji will be used as a hypetool.
Reason: Gorosei need to look menacing.
Having said that Sanji can still look cool by having some leadership feats (ordering Franky + the rest to escape) and some speed feats. He could even get some praise by the Gorosei, doesn't need to be done to dirty.
But all in all we know the role of Sanji lex Arlong, Satori, Enel, Vergo, Doflamingo & King.
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Sanji will fight Saturn, and do some damage, before ending up helping Zoro with Kizaru, while Luffy fights Saturn.
Once Saturn attacks Bonney, and Sanji realizes that Bonney is a little girl, and that Saturn is the one who killed Kuma and ruined Bonney's life, Sanji is going to go wild.
I believe in Sanji, I just don't think he can 1v1 Saturn, or do enough damage to seriously wound him.
Impossible. If Zoro let's Lucci loose 3-7 Strawhats die. Lucci is way worse than Kizaru who only cares about the mission and the big goals. Lucci would not hesitate on killing Usopp, Nami, Chopper and so on.
Luffy knew all the way back in Enies Lobby to not take his eyes of Lucci, Zoro done the same in Egghead.
Only two ways Zoro can fight Kizaru
1 - He defeats Lucci quickly (unlikely atm) and proceeds to fight Kizaru
2 - Swap. Luffy vs Saturn, Zoro vs Kizaru, Sanji vs Lucci
Edit: Well, there might be a option 3 if Jinbei could take Lucci of Zoros hands. Then Zoro and Sanji could team up against Kizaru I suppose.