You don't need fan fiction. Martin has plenty of other stuff that he's written in teh ASOIAF world. Dunk & Egg is a good place to start. Also, the World of Ice and Fire is an awesome book that really fleshes out the universe that he's created. Honestly, that's the most impressive thing that Martin has done, in my opinion. He's created this world that is just incredibly detailed. After having read a lot of the history, when you re-read the books you see that nothing is done on accident. Almost every character has a backstory or familial history, even minor ones who only show up for a page or two. Even if Martin fails to complete the two final books in the series, we can look forward to people creating tons of content set in the universe that he created. His wordbuilding ability is incredible, and in my opinion is rivaled only by Tolkien and Star Wars.
