Zoro killing Kaido, Sanji fighting Katakuri, and Shanks being Xebec's son are all reasonable theories.
Sanji fighting Akainu is stupid.
As for ZKK, since you mention it twice:
Oda just made it so that Zoro scars Kaido but gets a dragon slaying against another dragon.
The fact Zoro slayed a dragon means that Zoro killing Kaido was always a reasonable theory.
Since there was only one enemy dragon up for consideration.
Nothing links Sanji and Akainu, though.
I don't think I've ever heard anyone suggest that fight matchup either.
So yeah, that one is the most stupid.