Only 3 Flying Six are left.
Black Maria is with Kaido and Sanji can't fight woman.
Who'sWho was with Queen last seen and he didn't chase Drake for some reason.
Sasaki was freed by his subordinates and he is outside Onigashima now. Maybe he would go to the roof to stop samurai.
Jinbei might stall Big Mom again. Oda likes to repeat this. First Jinbei pushed Big Mom out of Thousand Sunny and now he threw her with Robin's help.
Sanji vs King has more chance to happen since King was the only one to react when Sanji freed Momonosuke even though Queen was beside King.
Sanji also set his attention to flying df users around King when he gathered with other SH.
It's not even close to a foregone conclusion, I do admit I was 100% wrong about Sanji not having any confrontation with King at all, so I admit that, but it's the fact that 3 panels drives up the assumption that it's a foregone conclusion and Zoro fans do it too.
For Zoro it will NOT be a foregone conclusion until he wears the bandana and goes full 3 swords. I also think Oda is setting something up unexpected for Zoro ( i dont know what), he's been with Killer, Kid, Drake, Apoo during all of this.
Oda is setting something very specific up with the worstgen
For Sanji we dont know until, also dont know if he is going to fight with the raidsuit or not. It also could very well be King
To your point that Who's who stuck with Queen and did not chase Drake, that's another person that can block Luffy and Sanji, Hawkins is still there as well.
I am 100% guessing and sticking with Nico Robin vs Black Maria
Oda does have something big in store for Big Mom and have her interfere with the war, she can also intercept them.