Here's how I see the series end. I think the downward spiral continues until Walt has lost everything, including his empire. In his desperation, he makes some mistakes blowing his cover and eventually a shoot out with the DEA. After being shot and laying there dying, we get a quick zoom into Walt's eyes and then we are suddenly transported back to Walt sitting in the doctors office receiving the news that he has cancer. The whole series is a quick thought of Walt in light of his received tragic news.
It's likely not going to happen, but that'd be neat.