So far the CP0 have played a very complicit role within the arc, merely acting as background agents to remind us that the world government does have stakes within the fight (with the weaponry production), carrying out their will.
It is within their interest to allow for as many of the pirates to be taken down as possible, and that Kaido wins. That does bring the possibility that they might join the fight later on. Moreover, since Sanji is predominantly running around aimlessly, maybe he will encounter them. Since Sanji has always acted as the background agent to the strawhats (Mr. Prince in Alabasta saga, activating the gates of justice in Enies Lobby, etc.) it is quite possible that he will reprise that role within this war, taking out a threat unbeknownst to the others.
Moreover, Usopp and Nami do not have anyone to fight, since Page One and Ulti were taken out. It is possible that them and Sanji will meet up and have to take down some other threat (CP0).
Going by this logic, it means that King and Queen will have to be taken down by Marco and Chopper, which is less than likely, so maybe Zoro will get healed by Marco and aid them against those two.
I feel like Sanji deserves a big hype scene especially as we did not get much of that in WCI, Dressrosa, Punk Hazard, Fish-men Island, or Sabaody. Especially as Zoro got a major upgrade and showed off his potential, I feel like we need Sanji to be the same.
What do you think?
Theory by potato_22