Well, notch mentioned that he'd put "Him" in sometime, though it's hard to tell whether someone's sarcastic or not when you're on the internet. I think adding "Him" would be a pretty good scare, but after a while the scariness would wear away, like with every other mob.
Maybe as a halloween-specific event?
Herobrine, or "Him", is a hoax made by some people who make a livestream of them playing Minecraft. Little did the viewers know that he retextured an iron door to look like a default player with empty, white eyes. He put the retextured door in a room, and then pretended to encounter "Him".
You can find more about it on the wiki.