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.