If Oda decides to make the Gorosei actual badasses he better give the appropriate focus to the one member of their club that actually carries a weapon with him. Like, I don't care if the guy is called Ghandi, mars or whatever but give the guy with a weapon respect.
If people want Sanji to give his stepdad as an opponent? Aight with me.
I am very unsure that Sanji WILL get a Gorosei opponent since I could easily see them getting divided up between other characters (Dragon, Sabo, Kidd, and or Law, etc) but if he has to fight anybody of them the mercury variant seems appropriate enough.