While HBO has no current plans for a spin-off series based on "Game of Thrones," author George R.R. Martin has shared some potential plot concepts that the studios could adapt if they ever decide to move forward with it. According to Martin, much of the material needed has already been published.

"The most natural follow-up would be an adaptation of my 'Dunk and Egg' stories," the author stated in a report from Entertainment Weekly. "Each of the novellas could easily be done as a two-hour stand-alone movie for television; that would probably be the ideal way to do them, rather than as an ongoing weekly series."

The 'Dunk and Egg' novellas are three short stories which have since been compiled in the book "A Knight of the Seven Kingdoms." The stories are set roughly 90 years prior to the main books and focus on a hedge knight named Ser Duncan the Tall and his friend Egg, who is later revealed to be King Aegon Targaryen V. 

However, these short stories have a much lighter tone when compared to "Game of Thrones" and the author states that if the studios wanted something of a similar tone then it would be better to focus on the Baratheon Rebellion that saw Robert Baratheon and Eddard Stark wage a war against the Targaryens.

A second suggestion would be to focus on the Targaryens when they first arrived, conquering the Seven Kingdoms and combining them into the empire that it is now in "Game of Thrones."

The author reiterated that the world that the series is set in is quite large and that there are multiple stories that could be picked for a spin-off series. There are many unexplored lands in Westeros, Essos and the lands beyond them that could easily be used for a new HBO series.

"Game of Thrones" itself will continue this year with season 6 premiering this April 24.