Well, he won't. Mihawk might be Zoro strongest opponent but definitely not the last one.
We know the final war is against the WG, we know that it will involve the ancient weapons and as Vegapank said and Koala recently reminded us the Pirate King, the one who will find the One Piece will have access to those weapons.
Now, Mihawk is in the race for the One Piece as part of the cross guild, even if this is against his own will.
Also Mihawk stating that PK is harder than surpassing him + Luffy stating that anything less than WSS in the Pk crew would make him look bad we can easily trace the lines and find out that Mihawk will fall before One Piece, so before the final war with Imu.
We can debate if Cross guild falls before or after the other yonkos, I actually believe they'll be the last ones just before they get the hands on One piece.
Buggy for the meme will be close and be Luffy last opponent before the One Piece, but do not expect Buggy to become PK and destroy 1000+ chapters when the entire personality of Luffy him saying he'll become King of Pirates + this would also go against Zoro and Sanji that are called the Wings of the Future Pirate King that will make him soar etc, imagine the entire crew, also Nami and Jinbe stating that Luffy will be that man and this not happening. Not possible, even though Luffy has an other deeper dream he has to become Pirate King and also get his hands on ancient weapons and fight against Imu alongside World Strongest Swordsman Zoro.