I think part of this primarily depends on how you approach your writing. If your focus is on the story whether it be an epic of good vs evil or a charater focussed journey then it is essentially 1 story (no matter how many books it takes to tell the story) and you might consider it a bonus if...
Your answer has highlighted a good point and perhaps a difference in approach to writing or creation than mine. My mind drifts to imagery and world creation very easlily but my thoughts and creations are very generic and , aiming to create and write about quite "classic" high fantasy worlds and...