This is a little bit out there, but based on Vince's promise of "no loose ends," the reappearance of Gretchen and Elliot, and interviews with the writer/director of "Granite State" that imply we aren't done exploring the Grey Matter plot line, maybe this isn't too far fetched.
We know Walt's mother is alive, his relationship with her is strained and several clues are given that mom and Skyler in particular don't get along. It's also implied that Walt's mom has money.
In "Four Days Out" Walt uses a visit to his mother as an excuse to go cook, telling Skyler something along the lines of, "she needs to know about my cancer, and I need to be sure if something happens to me you and the kids will be taken care of in her will." This implies mom has a significant amount of money - though we're never told how much, it's enough that Skyler buys this as an excuse to visit. When Skyler drops him at the airport and Walt tells her she can call if she needs him, Skyler brushes it off and acts like there is nothing in the world that would make her call. I think Walt used the dislike between Sky and his mom to his advantage, otherwise he'd be taking too much of a chance using a visit as an excuse for a solo trip. In most families it would likely result with a suggestion for everyone to go, an opportunity for the kids to visit granny - but Walt knows Skyler hates his mother and would never suggest this, so it is a great cover for a four-day meth cooking binge.
We get another callback to mom in the episode when Skyler confronts Walt with all his lies and kicks him out of the house. When she is listing off all the things she's investigated that don't add up, she mentions that she called Gretchen and found out that they aren't paying for Walt's treatment. Her next thought was to call Walt's mom - prefaced by a sarcastic, "thanks for that, by the way," indicating her deep hatred for his mom to the point that she can't even stand talking to her. But Sky calls anyway because, "I don't know, I thought maybe when you visited her you worked something out, that maybe she agreed to help you." (Another clue she has money - at least enough that she might have paid for costly treatment).
I think in Felina it will be revealed that Walt's mom is extremely wealthy because she bought shares in Grey Matter when Walt was still a partner, and because she disagreed with the way Walt departed the company and/or Gretchen she refused to sell her shares even though he asked her to. This is the source of their estrangement, and likely will have implications for why mom/Skyler hate each other so much. My guess is that Walt's mom loved Gretchen and sees Skyler as the reason Walt walked away from her.
Saul goes out of his way in "Granite State" to remind Walt that there is no way he can simply give his money to his family without the feds taking it.
But what if Walt's mom were to die? Even if she hates Skyler, surely she would leave her fortune to her only grandchildren?
I think somehow Walt uses the ricin on his mom, whether during a last "making amends because I'm dying" visit or by putting it in some food or drink he knows she loves and mailing it to her with a letter of apology. An old woman dying of flu-like symptoms would not raise any red flags, nor would the inheritance if it is her only family, and the feds could not legally RICO it because it would be "unimpeachable."
The writers have telegraphed the Lydia-tea-Stevia thing on purpose as a red herring. It is too obvious. Lydia will die, but not by poisoning.
Other than that, I just hope Jesse makes it out alive.
