Is it possible that Corvo may really be Emily's father? I know that it seems like he's a father figure to her, but maybe he actually is her dad? There seems to be some hints here and there towards that being the case.

Corvo could be Emily's father, though it's more likely that he's the Empress' lover after her husband died and a father figure to her.

Obviously, though, SOME people suspect it to be true.

Given he's the omnipresent male figure in her life, I'm inclined to think it's an irrelevant question.

there's already a thread exactly like this somewhere...

Well, if you read some of the in-game books, you'll learn that Lord Protectors are chosen by the Ruler they'll be protecting when the Ruler is 12 years old. Many people in the game comment about how young Corvo is, so we can assume that Empress Jasmine selected him to protect her when he was only a little older than herself, meaning they knew each other as children. It isn't out of bounds to think romance might have entered into it.

Second, as far as I know, NO ONE in the game ever mentions the Empress having a husband, or there being a former Consort. Not even in any of the books I read in game. However - Empress Jasmine's father IS mentioned in a book. And he is a Kaldwin. And she is a Kaldwin. And Emily is a Kaldwin. Not sure how surnames are handled in the world of Dishonored, or among royalty, but it could imply Emily was conceived out of wedlock, since she got her mother's surname instead of a father's surname.

Third, at one point Emily draws a picture of Corvo and lables it "Daddy". It could be simply that he is a father figure to her and she has adopted him as such, or it could be something more.

Lastly, in the Golden Age ending of the game, when Corvo eventually dies, Emily has him buried beside her mother in her mother's royal tomb. It could be that Corvo got the Empress Jasmine pregnant while they were teens, it was a scandal (since he was a foreigner) and it was kept a secret from the populace who Emily's father was. Members of the court may not have mentioned it out of respect for the royal family - so it would have been an open secret that perhpas even Emily herself was aware of. She would have been encouraged to address him by his name. After the assassination of the Empress, no one trying to use Emily in their power plays for the throne would have wanted the information to come out that her father was the "killer", as it would have tainted her claim to the throne.

Just theories, and like others have said, it is very much an open question. Regardless, the game makes it very clear that biological or not, Emily considers Corvo her father.

EDIT: spelling