That's my thought on it. I think Shanks is "good" but teeters on the morally grey side of things. To me, I think he believes in Luffy but he and Luffy may entirely have different ideals on how to achieve their goals for example.
To me this would also make him a great "antagonist" if Oda decides to go that route with the character. Imagine Luffy misunderstanding his respect for Shanks or Shanks misunderstanding Luffy's beliefs. I could see a very emotional conflict spawning from that.
I'd even go as far to say he could be the final "antagonist" or battle, circling the plot back to chapter 1 and Luffy surpassing his mentor. He has a small group of pirates just like Luffy and BB. Add to the fact that Mihawk technically has to be Zoro's final battle and could partner with Shanks for that exact reason. Idk, I'm part of the minority with this thought so who knows.
For me he is the final antagonist to surprass roger. Because imo roger = shanks, rp = rhp (and garp = dragon).
But luffy has to surprass roger because he has to achieve something which roger couldn't. As it is the job of joyboy aka luffy.
To defeat imu + gorosei.
Imu>gorosei>roger,garp,wb,admirals etc.
Otherwise it just doesn't make sense, how for 800 years the tenryubito didn't fall. I mean, if the admirals were the strongest the wg has, then what if they came to the idea to get rid of the tenryubito?
What could stop them? Nothing.
The reason why they rule for 800 years is simple, imu>gorosei>admirals.
Which makes sense, since the ancient kingdom fought and fall... against whom? Marines? The kings and armies of the 20 kingdoms? Unlikely. In op numbers means nothing (haki), so there has to be people who are strong enough.
That is why, I believe imu = joyboy (aka luffy eos).
And that the marines wouldn't even be the final antagonists at the end of op, where the whole world will be at war. I can't see the admirals, va etc serving the tenryubitos 🤷♂️
So at the end, it will be the tenryubitos vs maaaaaaaaany maaaaaany people, led by luffy.
Imo.