Walt cooks meth one more time.
Of course he does! How could he not? The final season and Walt never does chemistry? After all, that is the price of Jesse’s life that he negotiated with Uncle Jack. They can’t bring it up and not follow through with the concept.
[Perhaps, though, he’s doing the final cook as a payment for the life of his brother-in-law? Or Jesse?]
So Walt does one more cook, but perhaps he alters the recipe a little, creating a lethal gas that would kill bystanders who don’t like wearing gas masks (i.e. Uncle Jack and Kevin Rankin’s character). This would be a call back to the first episode and the death of Crazy 8’s partner in the RV. It’s possible that it would just maim, and not kill some of the Aryans (“She blinded me… with SCIENCE!”). It would also set up sociopath Todd (who wears a gas mask because Mr. White told him to) to be a new archenemy, perhaps one who would force Walt to call the vacuum salesman that Saul knows.
But in any case we have to return to Walt’s strengths — his science background — for the epic climax of this story.
As for 52 year old Walt, armed with a huge gun and a small bottle of ricin… I got nothing.