Sanderson's already said that he'd never do it. His writing style is too different and he's not prepared to go that dark (note how he apologised to his hardcore Mormon fans for the - extremely mild - sexual references and swearing even in WoT), and not going that dark would not fit in with GRRM's style.

GRRM's always said he will finish the books. If he was in the Robert Jordan boat - learning years ahead of time that he was on his way out - he'd make alternate plans (and I suspect that Daniel Abraham is the only author in a million years he'd consider letting within touching distance of ASoIaF), but beyond that he will not allow someone to finish the books with no preperation after his passing, and he will not allow someone else to finish the books whilst he is still alive.