There is no set up just to scar kaido
The set up for the one who opens the scar is the one kaido thinks is the one worthy of killing him.
This is actually whats said and this is the whole sequence of that part,
It starts of with doji outright saying this fight isnt over until his head is severed. I e this war wont be over until hes dead.
so kaidos words
I saw a visage of oden
I could have let you kill me
But you are not oden
We will never see a monstrous samurai like oden again
Its too shallow you dont even have enough power to open the old wound
people just focus on his last line instead of his whole speech and just say zoro will just open the wound without looking at why kaido said that,
kaido was going to let the scabbards kill him until he realized they are not oden they are not as strong as oden they couldnt even open the wound oden gave him
so the samurai that CAN open the scar that IS as monstrous as oden IS the one he will accept that kills him.
The set up isnt for there to be someone to just open the wound, but the one who does that is capable of killing him. And thats kaidos own view.
To make it simple so you can understand
kaido is saying YOU CANT KILL ME YOURE WEAK YOU CANT EVEN OPEN THE SCAR
so whoever can scar him is strong enough to kill him
To top it all off he is carrying odens will in enma
and odens will was to take kaidos head