I love Sanderson, he is my favorite fantasy author, and you are correct about his marvelous job finishing WoT, but he is just not the right fit tonally for ASOIAF. He doesn't even write swear words, and GRRM's books are fairly liberal with the fucks and the cunts. I would however be in favor of Joe Abercrombie or someone with a bit of a darker bent to their writing taking over if needed.