I wouldn't mind harder dungeons... But I do think there's reason to have easy enough dungeons that everyone can do them.

I think a harder (and more rewarding version) of the dungeons would be a valid option, or at the very least one where you had boss mechanics that don't end up being kind of optional, if the devs are willing to do that. I don't know how long it takes to create a dungeon boss, but I don't know if creating 2 versions of a boss would increase the workload all that much.

If they ever do decide to make harder versions of dungeons, I do hope they don't go the route of some MMOs of just increasing enemies HP and Damage to try making them harder...