Okay, here's an idea I had about what could happen in the finale, although I admit this theory sounds a lot like a super-villain plot:
Remember how Gretchen and Elliot from Gray Matter announced the opening of a new drug treatment center in Albuquerque on Charlie Rose in the last episode? Well, there's bound to be some news cameras there, right? So what if Walt drives up in his car to that event walks up to that grand opening brainwashing his M60 and maybe firing off a couple of rounds? And initially you think "He's gonna kill Gretchen and Elliot? Why? Isn't that a little late in the game?"
But no, after he forces them to stay, he then tells the news camera people to go live and nationwide. Then, in a deliberate echo of the very first episode, Walter identifies himself, confirms that he is also the co-founder of Gray Matter and the drug kingpin Heisenberg, and that, because he doesn't have much long to live due to his cancer, he's going to finally bare all. He reveals the real reason why he left Gray Matter and, because he choose to walk away, it was the biggest mistake of his life as his co-founders have profited off his ideas. And that now, with his Blue Sky meth, it's happening all over again, but this time he's going to set the record straight. He then outs Todd, Jack, and Lydia, maybe even showing their photographs, saying how they have "stolen his life's work" and are making millions off of his name, that they're responsible for the murder of Hank, etc. thus telling them they, like him, will now have no place to run or hide. Finally, he apologizes to his family for all that he has put them through and that they shouldn't have to suffer for what he has done. Then he takes the ricin pill and his last words are "Remember my name."