I don't think he could be the main villain per se, but I think he could be Luffy's main fight before the War begins and turns into chaos fighting against "Rocks". I personally still don't believe Luffy and Kaido will have a proper mano a mano fight, not before Blackbeard or Akainu. Throwing Big Mom into the mix just makes it too much. There will be other factors involved that make them go down. This is why I personally believe there is an Ace, not to mention:
- The fact that Tama/Ace (in relation to the name only) is a thing this arc that has yet to pay off. My guess is Tama will somehow be present for Luffy v. this Ace, and Luffy will say "Don't you dare use my brothers name!", revealing to Tama that Ace was Luffy's brother. The alternative would be a whitebeard pirate or Izo calls him Ace's Brother and she hears it, but I think the first option is more dramatic.
- Luffy ALWAYS warrants a major mano a mano fight every arc. Kaido almost can't be that for him. If Oda has him win alone, then Luffy will become the strongest "creature" in the world by default after this arc. That reduces the stakes of future arcs. I'm not talking about an attack Zoro or Kidd may deliver, but a full blown teamup is likely warranted to take down either Yonko.
So yes, I do believe this arc will have its "Katakuri" before the Rocks plot truly kicks in.