Smoothie is a complete non starter. Zoro has nothing to do with the Big Mom Pirates. What's the point in having him roam the country seeing the horrors of the Beast Pirates oppression then to fight someone that wasn't even slightly involved in it?
No, it has to be a Beast Pirate that Zoro fights. Orochi, an idea that was increasingly laughable as the arc went on, was totally shattered when Kaido lopped his head off. On the native Wano side, there's only Fujurokujo and Hotei, who are basically a step up from nobodies and to be frank don't cut it. I'd honestly expect them to be oneshot like Hyozou if they tried to fight Zoro.
As for what Beast Pirate- Zoro will play a role in defeating Kaido. I don't think Oda has been overly shy about foreshadowing that. When he got Oden's sword that was a guarantee he'll at the very least land a hit on Kaido. Tied with the Ryuma parallels and Zoro being placed along with Kid and Law in the past few chapters (other characters who will play a big role in defeating him), an increased focus on the Luffy/Zoro partnership throughout the arc, Zoro's links to Wano and its people- it's going to happen. The question then is how much of a role will Zoro have in the final fight with Kaido?
My thoughts prior to this chapter is that Zoro will defeat King, be totally fucked, land one big hit on Kaido and that'll be it. If I had to pick, that is still what I'd predict will happen, regardless of Sanji vs King this chapter. The absolute most I can see happening with Sanji and King is that Sanji plays backup to someone else landing the final blow on him. That someone could well be Zoro. I don't want this to happen, because to be frank I think it would suck for Sanji. King is the opponent who simply makes the most sense for Zoro, based on everything Oda has written in the past. Yes, we've yet to really see King use his sword- but we've yet to see him fight seriously at all. It's when the hybrids come out that we know the real fights are beginning, and that still looks to be a fair bit away.
Queen... I just don't see it. Queen just doesn't give me a Zoro fight-vibe. I can't really see what the point in Zoro fighting Queen would be, when King's around. A fight over a woman, Hiyori? That's the setup for a Sanji fight, not a Zoro one. Oda doing that instead of getting revenge for Udon for another character? Not seeing it.
The thing I can see, if Zoro vs King is not to happen, would be a quick fight with Who's Who before moving on to playing a large role in defeating Kaido. In which case I think Marco would be beating King, not Sanji. I'd rather this didn't happen- the knock-on effect of this would, I feel, essentially shaft most of the Strawhats. But it could happen, especially if what Oda is aiming for- and this wouldn't be much of a surprise- is a Worst Generation heavy finish.
