Reddit comments:

So whos up for a game of thrones movie detailing the conquering of the seven kingdoms and history before the show?

Balon's first Rebellion, the Blackfyre Rebellion, the Dance of the Dragons...

The possibilities are endless, Roberts Rebellion, Dunk and Egg, Aegons Conquest, Doom of Valyria...

In an ideal world HBO would produce these and feed the profits to fund the TV show.

More fodder for Game of Thrones prequels:

http://pandawhale.com/post/6285/game-of-thrones-prequel-fan-fiction-kickstarter-project

http://pandawhale.com/post/41050/game-of-thrones-prequel-infographic-from-oberyns-dialogue-with-tyrion-season-4-episode-1