Possible endings:
Jesse finds out about Brock (or Jane) and kills Walt. Probably with the ricin cigarette. Or turns state's evidence, sending him to prison. This would work from a "cathartic" perspective, as Walt would finally get his comeuppance for some of the dastardly things he's done, primarily to Jesse.
Walt sacrifices Jesse in order to save himself, either killing him, ordering (or somehow facilitating) his death, or framing him and sending him to prison in Walt's place. This would work from a "Walt's ultimate fall to the dark side" perspective, because while he's profoundly betrayed Jesse on any number of occasions, he's always shown SOME level of loyalty to his partner as well. This would represent the ultimate severing of that bond.
Walt ends up a schmuck, either teaching again or working at the car wash, which works because it brings the narrative full circle, leaving Walt approximately where he was at the beginning, and also has an air of comeuppance to it as well, as at this point Walt's worst nightmare would be going back to the ineffectual sort of existence he once had.
Walt ends up on top, but loses his family in the process. A "these are the true wages of sin" kind of thing.
Regardless of the details, I think the eventual ending will include elements of one or more of these theories. And any one of them, in its way, I think, would be a "fair" ending. Which is not to say that any one of them will satisfy everyone (or even anyone, for that matter).
My own pet theory is that somehow the ricin will come back into play, because Walt has been plotting to kill SOMEONE with the stuff since the beginning of season 2, and it never ends up happening. It's like the ultimate Chekov's Gun of the series... the damn thing has to fire at some point.
