It's like the writers completely forgot about that after season 5. It'd have been better if they'd returned to it later, particularly since Dawn never had any really interesting or memorable story lines.

During and after season 5. It was the same "add a kid" gimmick all the shows used back then. Great set up and then nothing.

Could have been interesting, but given that she was the "key" to a hellish portal, it didn't really need to extend beyond that. What other powers could she possibly have possessed?

^I've seen a fan fiction where she has the ability to open portals to all dimensions, not just hell dimensions. She only uses it when needed though because it injures her.