The relationship between Leto [Paul's father, that is] and Jessica is an epic relationship of true love and adoration for each other. One of those rare loves which... let's face it... was probably destined anyway but Bene Gesserit imprinting doesn't hurt.
But that's not the point of this post.
The point is this: They loved each other so deeply but were unable to marry because Duke Leto had to keep the possibility of alliance through marriage open. We all know this.
It occurred to me last night, though: If Leto and Jessica found out, after they were so much in love, that she was the daughter of Harkonnen... wouldn't that mean that suddenly she would be an heir to a great house and therefore marriage material to the Duke?
The melange hitting the fan due to such a revelation would be ... large. But that aside, how's that for a tragedy when they're not just star-crossed by fate, but also by ignorance?