After the anticlimactic end to “Game of Thrones,” HBO has been searching for that series that can not only fill a place in the schedule but also become a part of the cultural conversation. While “Westworld” seemed like the heir apparent, the most recent season failed to really grow the audience. And now, HBO is looking at the return of “His Dark Materials” as the potential epic action series that can become another jewel in the crown of the premium network.