My Future Sight said that Sanji's final opponent would be Gin. Gin said that he will met Sanji again somewhere in the future, so in order to build hype for that fight, Sanji has to beats as much opponents as possible (Pizarro/Lafitte, CP0 leader, SSG, Vegapunk, Greenbull/Kizaru, Blond Gorosei) then Gin.
It will be a nice plot twist if Sanji turns out to be the one to fight Mihawk, while Zoro will fight Shanks instead.
And this theory is supported by their difference of height, as was pointed out in another thread.
I can see other possibilities, and this is how I see them in terms of which one is more interesting than the other as Sanji's last opponent:
Mihawk > New character > Kizaru > Gin > Green Bull > Akainu
The problem of Sanji fighting any character below Mihawk among the ones already introduced is that it's already too obvious that Sanji is stronger than them, especially Akainu; Aside from some delusional Zoro fans, it's too obvious to everyone that Sanji's flames are much hotter than Akainu's magma, so their fight wouldn't even be fun.
We don't know much about Green Bull, but Doflamingo spoke as if Green Bull were comparable to Fujitora in terms of strength, so he's probably not very strong.
And this theory is supported by their difference of height, as was pointed out in another thread.
I can see other possibilities, and this is how I see them in terms of which one is more interesting than the other as Sanji's last opponent:
Mihawk > New character > Kizaru > Gin > Green Bull > Akainu
The problem of Sanji fighting any character below Mihawk among the ones already introduced is that it's already too obvious that Sanji is stronger than them, especially Akainu; Aside from some delusional Zoro fans, it's too obvious to everyone that Sanji's flames are much hotter than Akainu's magma, so their fight wouldn't even be fun.
We don't know much about Green Bull, but Doflamingo spoke as if Green Bull were comparable to Fujitora in terms of strength, so he's probably not very strong.
