And why is that nothing? Mihawk has achieved success and now he wants somebody else to succeed him. He is perfectly fine character.
In fact, Shanks feel like somebody who inspired Luffy, a Jesus figure. While Mihawk feels like an actual person.
Why does he need to be in the main story? To be Luffy's cheerleader?
Because he has little to no agency. He barely feels like a character just like Shanks
He's a character with motivations that just so happen to perfectly fit the needs of Zoro as a character.
If all he does is fight Zoro at the end then all that does is prove that there wasn't much more to him to begin with and unfortunately that's likely since there isn't much else to his character. He wants to fight someone stronger than his old rival, no?
Well all he has to do is train Zoro then sit on a random island waiting for him to come back to scratch that itch
I want him to be in the main story so he's more than just a goal for Zoro