I've struggled with the same question.
1) It's hard to think of something original. It seems everythings been done. But if you COULD think of something no one has thought of yet, it might sell pretty well.
2) Though you could create something that is not original, would it sell if they could get it elsewhere for free? example: how much in sales has vB made, since the rise of phpBB?)
*shrug*
If you went with idea #2, your new addition would have to be outlandishly useful or unique for people to want to purchase it.
good luck to you
--cheers