To be honest, I really don't like the Discovery feature. It's too vague and doesn't seem like it has any direct use, like the other background features do. I really don't know what I'm supposed to do with it.
"It might be a great truth about the cosmos, the deities, the powerful beings of the outer planes, or the forces of nature. It could be a site that no one else has ever seen. . . . You might have uncovered a fact that has long been forgotten, or unearthed some relic of the past that could rewrite history."
As to what you're supposed to do with it - it seems to me that you're meant to give the player some information that will be important to the campaign. That's its use. (In some campaigns it would make sense for the player to make something up that the GM then incorporates - but I think the GM-led approach probably works better when you're running a pre-authored adventure.)
Ravenloft has lots of secrets, doesn't it? The story behind Strahd's fall and imprisonment. The location and nature of the Sun Sword. The motivations of mysterious fortune tellers. The secret for escaping the Mists. Etc, etc.
I would have though that one of these would make for a pretty good discovery.
Is there a way I could phrase it so the player has to think about it a little bit? So it's not so meta?
I don't think you need to make it very obscure. Other backgrounds give reasonably clear advantages - there's no reason why this should be different.
If the character has drug-induced visions, he could have had visions of (say) escaping through mist via a certain path, or involving some sort of mysterious incantation. At the start of the campaign, the character doesn't know why this is significant (and perhaps the player doesn't either, depending on how much he knows of Ravenloft lore), but it's written down there on the PC sheet. So once the campaign moves to Ravenloft via entrapping mists, the player (one would hope) will join the fairly obvious dots, thereby enabling his character to do likewise.
I don't think other sorts of discoveries - visions about Strahd, or the Sun Sword, or whatever - should be any more difficult to put into the PC's backstory.
The key thing is that the player knows it's his discovery, and so (again, one hopes) will be ready to see it come to light in the game. So if the discovery involves visions of (say) a shadowed figure on a castle parapet, the player should be looking out for parapets fit for brooding, and recognise Castle Ravenloft when you describe it to him.