okay, I'm predicting it now... it hit me on the can last night...
I think the ricin is how the series ends - Walt will poison BOTH himself and hank. They'll haev a man to man sit down, he'll confess to everything, poison the shot or coffee they share, and then they'll both collapse shortly thereafter.
Walt can't come at the better trained hank with that big ass machine gun, and they're developing Hank to be his mighty nemesis at the end.