IF BB kills Shanks, I don't think Mihawk will be a part of it, tbf.
BUt idk man...Shanks has some mysterious build-up with BB since forever. He gives up his arm betting on Luffy. He's excited about Luffy. He talks to Rayleigh about Luffy saying Luffy said the same thing Roger said. I believe this "Shanks is evil" thing is complete bait. A plot twist just for the sake of a twist. Luffy promised Shanks that he would find a crew that would surpass the RHP's, find the One Piece and become Pirate King.
There are 2 ways for it. Luffy either beats Shanks directly, or he beats whoever beats Shanks. You know what I mean?
BB is basically an anti-Luffy. His introduction is literally find something delicious that Luffy found awful. And finding awful something that Luffy thought was delicious.
Making Shanks evil and the "real last boss" at this point, would be like Arya killing the Night King instead of Jon Snow. It's just to SUBVERT the expectations people had since forever about these two characters and how they would interact with Luffy during his quest.
I don't think he's evil, that's the thing. I think he misunderstands Roger and Luffy's secret dream. I think that's the issue with a lot of people here when it comes to doubting it. It's not about a plot twisty, purely evil Shanks. It's just about Luffy and Shanks' goals not aligning, that's all.
I also disagree with the GoT comparison. Fighting Shanks has been heavily foreshadowed since PH. Luffy WANTS to fight him. Shanks very clearly does too. Funny thing is Whitebeards speech in Marineford before he died did a lot of damage to Blackbeard being the final "everything" in this series.
He basically outlines the final plotline, which aligns with Roger's own views on it after discovering One Piece.
- Said person will find One Piece and find out about the world. Whitebeard believes this is not Blackbeard. Roger did this part only.
- Said person will bear centuries of history and weight on their shoulders and challenge the world. Roger failed at this part because Shirahoshi wasn't born yet. It wasn't him either.
When you put two and two together, it very directly seems to imply that Blackbeard isn't the endgame here, just another major obstacle set up by the plot. Now for Shanks, that comes with a bit of faith on there being far more to his character than meets the eye. Him being a year older than the God Valley incident and Roger allegedly having him since he was a baby seems to imply....things, but we have to wait and see.
There's also the notion that Oda purposefully hid a deep conversation between Roger and Shanks, which caused Shanks to cry (Oden witnessed it). Why did he hide this? It adds to the mystery surrounding Shanks.