I always (and others as well) had an inkling that Nightingales would return. I feel that would be a great franchise house and I'd like to see what would be next for them.
Because other people mentioned artist they'd like to see work with HHN, one in particular comes to mind. I think it would be really cool to see something from Jhonen Vasquez. His art style is dark and so are his stories/shows. He created Invader Zim, which was a kids show, but was extremely dark. One episode involved the main character, an alien named Zim, searching for ways to look more human in an attempt to blend in with his classmates. His consensus was to eat and swallow the organs of children at his school (because aliens didn't have human organs in this show). At the end of the episode he becomes so bloated with the organs of children at the school (or Skool - yeah that's actually how they spelled it ), that while speaking to someone, an intestine falls out his mouth and he quickly slurps it back up and acts as if nothing happens. Classic Nickelodeon is where it's at. Also, the show got canceled because in the final episode (which I don't think aired - I could be wrong on this), he actually killed his greatest foe (who was of course a child - they were all at Elementary level I believe).
Now I'm not saying an Invader Zim house, but another Vasquez story that is wonderful is Johnny the Homicidal Maniac. With a title like that, need I say more? This would translate well in my opinion.