I can't think of one off the top of my head, but I'm sure it has definitely happened. Two of the more famous recent book adaptations(LOTR, Harry Potter) have developed or are working on prequel series. I honestly prefer a prequel or a sort-of unrelated earlier story as the next series(e.g. Dunk and Egg) rather than a sequel.
I am just a little worried that they'll **** up the sequel without significant source material, and that it would hurt the GoT/ASOIAF story a bit as a whole. If it's a bit more in the future, that might play a little better IDK.
As for a direct prequel, they've said they're not doing Robert's Rebellion, which I think is good. Part of what Martin is trying to do throughout the books(as well as in the show) is tell the story of Robert's Rebellion through the thoughts, actions,etc. of characters in the current time. By the end, you basically know what happened, it just doesn't seem right to do that series.
True, but I think they could have extended the material they had already present into a couple more episodes. It would have made things feel a bit less rushed, and provided better buildup to the high-action/dramatic scenes in this seasons.