So, after rereading a couple of important chapters, it doesn't seeem unlikely that Shanks talked about Luffy, but it's most likely Blackbeard. The point is that he's not evil, even if it was Luffy. That might've been made for us to question his allegiances, but he's not going to turn out to be evil, I strongly believe.
Shanks has been heavily influenced by the norse god Tyr, who lost an arm to fenrir to bring balance to the world. His boat and overall scheme is very nordic, and Oda has touched on several different kinds of religions/beliefs/myths, often intersecting them or blending them into one. I used to think that we're running out of time for Elbaf, but now I'm all in on it, I think it's a missing piece in the Ancient History puzzle, and likely where Luffy vs Blackbeard happens. I think that's happening and that's where we're gonna get to know how Shanks ends up. If Oda keeps his mythological references, he dies at the hands of Blackbeard.
Here's my main point: I think Shanks has the last road poneglyph. It was clearly mentioned in Oden's flashback that there was a Road Poneglyph in Fishman Island. The lack of colour scheme didn't help, but the anime pretty much confirmed it. There's no way in hell Oda would've let such a massive thing get into the anime if he didn't plan for that to happen. So.. we have 1 poneglyph left, who was previously in FI, and no idea where he is. There's only one relevant character who knows about it, probably knows a lot about the ancient history, and is powerful enough/secretive enough to have it hidden, and that's Shanks. I honestly think he got his hands on it, keeping it for Luffy, in case he turns out to fulfill his expectations, and proves himself the man Roger was waiting for, or someone else does.
What I'm honestly not sure about is his position towards Blackbeard. He has advised caution against him, but he never really seemed to directly oppose him. I honestly think he might be okay with Blackbeard, because he understands he might also be one of the candidates to bring the change of the world. I think this part is anyone's guess really.. if he's actually a personification of justice and fairness, so to speak, I could see him wanting no part in directly opposing Blackbeard, and having Luffy/Blackbeard duke it out.
On the other hand, it even seems more likely that he's against Blackbeard being the one Roger was waiting for, kinda like Whitebard, and advised the Gorosei about him, because he might believe the world Blackbeard wants to bring is even worse than the one they live in.
So, regarding the question on the thread: I'd say he's definitely not evil, but he might be more impartial than good.