Admirals are never going to be main arc antagonists.
I can see it happening against Nasujuro when Zoro forges Sandai into a black blade and breaks/defeats Shodai of Nasujuro to cement Sandai as the greatest Kitetsu blade.
The thing is, from a story perspective, readers expect Zoro not just to forge a kokuto and be done with it. He needs to learn to use it at its best.
So it can't be forged in his last fight with Mihawk.
It could be forged in his second-to-last fight, which will likely be Shiryu. The kokuto is the PU to defeat Shiryu, then, next fight, Zoro goes all out with aCoC + kokuto to claim the WSS title.
I've always pictured if not a fight then at least a skirmish between the monster trio and the three pre-ts Admirals. Luffy takes on Akainu for Ace reasons, Sanji Kuzan for the Robin matter, and Zoro Kizaru for the Ray parallels. The M3 doesn't have to defeat their opponents, just to push them back. Then the Admirals can become main fights for Dragon/Sabo/whomever else.
Point is, in this scenario I've always pictured Kizaru's light sword nicely contrasted by Zoro wielding a black blade. The visual alone makes it interesting.
But then again, Oda hasn't been a very logical guy when it comes to fight progression or even PU mechanics.