1.King is confirmed to be a swordsman iirc
2.The only relevant samurai in Wano are the scabbards and Kyoshiro. They sure as hell won't be and aren't near King in strength as far as Yonko FMs are portrayed. And on top of that King is a Zoan. If he's already stronger than them as fighters in base then the gap gets higher in his hybrid form.
We know Zoro's dream. Defeating King brings him closer to that dream. Just like defeating Kaido brings Luffy closer to his. King will be an obstacle for Zoro to overcome. It's as simple as that, any other layers besides that will be added on by Oda as the arc progresses.
I mean you never know, maybe Kyoshiro will end up being Zoro's final opponent. But I doubt that as Orochi currently trapped and destroyed every bridge in Wano except the one that connects the rest of the country to the flower capital where the guy is currently guarding. Kyo might be going down early lol.
Either King or Kyoshiro are the likely candidates to be Zoro's limit pushing fight. But King is the stronger of the two so I'm betting on him right now.
1. Diamante was a swordsman too.
2. Agree.
Don't get me wrong, as much as i love Zoro and King, i don't want this fight to happen for few reasons:
1. No foreshadowing ( I hate fights "by chance" in OP, one and only foreshadowing for that fight is that King is a swordsman ).
2. Irrelevance of YC ( let's be honest, YC are not threat for MC anymore. We have at least 8 YC or more fighters in Alliance side and one man that stronger than any YC. It may change, then i will rewatch my opinion ).
3. Hypothetical participation of Zoro in fight with Kaido ( it has more foreshadowing than Zoro vs King ).
4. "The second Ryuma" stuff. ( OP Magazine said that "The Second Ryuma" is born and now in Wano, anyone with a brain know that it's Zoro. I want Zoro to shine, i want Zoro to prove all the the Wano that he is exactly that "second coming". I bet that half of Wano even don't know about King's existence or barely care about him and beating "some underling" won't prove anything ).
These are not all the reasons, but i think you understand.