The overwhelming theme to my mind concerns the relationship between fiction and fantasy. Note how the story itself is called a "legend" by Washington Irving. This introduces us to this thematic concern. Legends by their nature are stories that may have an original basis in truth but have been elaborated and exaggerated over the years to become a very different kind of tale. It is important to realise how Irving creates a setting that stimulates the imagination of all those within it to such a great extent:
Certain it is, the place still continues under the sway of some witching power that holds a spell over the minds of the good people, causing them to walk in a continual reverie. They are given to all kinds of marvelous beliefs, are subject to trances and visions, and frequently see strange sights, and hear music and voices in the air. The whole neighbourhood abounds with local tales, haunted spots, and twilight superstitions...
If ever there were a place that could be used to explore the tenuous relationship between fact and fiction, then this imagination-saturated location is it.
Ichabod Crane, as an outsider to the community, is shown to be completely seduced by the powers of the imagination and his dreams. Note how, when he first visits the Van Tassel home, he is entranced by the bounty that he sees and falls victim to "sugared suppositions" of his future life being married to Katrina Van Tassel:
...and anon he passed the fragrant buckwheat fields, breathing the odour of the beehive, and as he beheld them, soft anticipations stole over his mind of dainty slapjacks, well buttered and garnished with honey or treacle, by the delicate little dimpled hand of Katrina Van Tassel.
He is a man who, as his dreams of his union with Katrina Van Tassel show, is unable to distinguish between fact and fantasy. Likewise, he is unable to see the story of the headless horseman for what it really is: a story. This makes his downfall easy in both situations. He has clearly got so far ahead of himself that his rejection by Katrina comes as a complete surprise, just as he believes in the reality of the headless horseman that haunts him. Thus the tale acts as a cautionary story, warning us to make sure that we keep very firm boundaries between fact and fantasy and never try to blur them.