For this question, I'm going have to go with The Mortal Instruments series by Cassandra Clare. I think there's so much to the Shadowhunter world that can be explored if it was turned into a tv series. There would be more time to explain the world, get to know the characters and understand the overall story. A spinoff series on The Infernal Devices would be even more awesome ;)
What book/series would you pick and thanks for visiting!