Indeed, producer Edward Kitsis has said they "have a very specific plan for that character,” and "if we do it right, it’s an a-ha moment.” Clearly, Henry's dad not a random nobody. So who could he be? What could lead to an ah-ha moment?
Right now, what we know about the mystery man is vague. According to Emma, he doesn't know about Henry, and she's also made it clear she doesn't think highly of him. Presumably he's not one of the characters we've already met, since a) they were almost all stuck in Storybrooke at the time of Henry's conception, and b) there's nothing to suggest Emma wouldn't remember what he looks like.
Our favorite theory is that Henry's dad is actually Baelfire, Mr. Gold/Rumplestiltskin's (Robert Carlyle) son. After all, we know Bae was transported to "a land without magic." He's the one major character who is unaccounted for, and we already know why he, unlike other fairytale characters we have yet to meet, wouldn't have been stuck in Storybrooke. It makes a lot of sense.
It would also be an interesting choice, story-wise. It could potentially force Emma and Mr. Gold to work together more. And given that Emma has hinted that Henry's dad is not the greatest guy, it could mean that Mr. Gold and Bae's eventual reunion is even more emotionally fraught than we currently expect it to be.
Of course, he could also be one of the new characters coming to the show in Season 2. Perhaps Captain Hook, since it would make sense that Neverland is separate from the Fairytale world. Hook could have found his way into our world for non-curse related reasons.
Or maybe they're planning to go in a different direction entirely. Maybe one day someone from our world will come to Storybrooke and cause trouble. The "ah-ha" moment could be learning that a newly introduced regular-human villain is Henry's father.
What do you think? Do you have a theory you like? Let us know in the comments below!