I actually hope they don't go that way, Walt is effectively irretrievable morally at this point because he has nothing left to lose thus making any sacrifice fairly weak. His family hates him now, he is definitely ID'd as a criminal, and he's on a short clock. While admittedly he could run off to a place with an absence of extradition laws and sexual mores and a surplus of beaches, that's not exactly a deep sacrifice especially considering the drug kingpin who's wife hates him has still never show any predilection towards debauchery anyway. Doing a good deed when you've got squat to lose isn't going to redeem him with any audience member who recalls that, especially if they also recall that Jesse is very definitely not a good guy. Everyone's been waiting for Walt to die for the entire show and season 4 and early five made it seem he was going to do it as arch-villain, I'm not sure where there going now but I think I might be disappointed if they go for sunshines and lollipops. Walt nobly sacrificing his life to save Jesse who is suddenly a good guy not a career criminal and murderer and goes on to spend his life doing charity work or something. It would be a decent ending but a kind of cheap. It would also not involve Walt taking ricin, which as you mention is the logical lead candidate for what happens to the stuff.

That was my thought to but it seems like a pretty serious Chehkov's gun for simple suicide of a character who is already dying and very easily set up to be simply shot. They went the extra mile with the bear/pool so I'm guessing there's a twis tn mind.