I think the writers didn't expect they would ever have to reveal why the key was valuable and open up the wessen/Grimm world. I see the key in episode 1 as a prop to "explain" why some guy with a sickle was trying to kill a dying woman. I doubt they thought Grimm would go 5 seasons. What will the treasure be? Building on the Indiana Jones story, maybe a very long life, while remaining forever young until the very end. This would give an opening for future Grimm books and movies--those in control may be looking for a cash flow down after this year.