An alternate scenario is that Zoro will defeat Mihawk first and take the WSS title from him then Shiryu will kill him before his eventual fight with Zoro. Still unlikely.
This is what I think too but the opposite. I personally think Luffy will achieve PK status before Zoro becomes the WSM.
I can see this play out like:
Luffy and Zoro eventually defeat Teach and his goonies after a long 5 days struggle. Zoro whispers to Luffy's ear and the other Shs are tearing up. The readers are dumbfounded because the expressions are a combination of joy and sadness.
Then we figured out that Zoro leaves the crew (temporarily). 10 chapters later, Oda cuts the scene to the ocean waves splashing against the rocky shores. Two figures among the hazy mist can be seen sizing each other up from a distance. Then camera zooms in closer and we see that these two individuals are Zoro and Mihawk. Then they clash with new and multiple named techniques.
Zoro, like Luffy, eventually succeeds his goal and becomes the WSM. Then Zoro rejoins the crew with everyone's delight.