Actually wouldn't mind Sanji vs Ben personally if Oda is capable of going that route
I wouldn't as well.
But I really don't mind Roux.
He's said to be extremely agile, have a great personality, more "upbeat", which usually fits Sanji's opponents more compared to Zoro's (only exception I can think of is Kuroobi/Hacchi).
Mfer executed that bandit in cold blood without Shanks ordering anything, and has been hyped since the beginning alongside Beckmann and Yasopp.
I wouldn't mind Sanji vs Roux not in the slightest.
The Sanji vs Beckmann is purely an alternate route I created in my headcanon because I had a really hard time trying to figure out how and when the fuck Oda would pull the Zoro vs Mihawk fight.
With Shanks declaring going after the One Piece, he legit jumped to "last boss" option for me. Having Mihawk there as Zoro's final opponent in the story would just be fitting.