Great question for the discussion board!
I just finished teaching this story to my American Literature II course and we discussed what could have happened to poor Ichabod!
In my opinion, Ichabod was "kidnapped" by Brom Bones and taken to an undisclosed location, where he was told to leave town or be physically hurt (I do not believe Brom Bones would have threatened to kill him) because he and Brom Bones were rivals for Katrina's affection. I do not believe that the Headless Horseman was real...I definitely think that Brom Bones was behind Ichabod's disappearance. After being threatened by Brom Bones, I believe Ichabod immediately left town without looking back!