A couple of things I have been thinking would happen for a while.. Walt will admit to Hank he is (was) Heisenberg, but will blackmail him by threatening to implicate Hank, who took the money for rehab. I've been thinking this since they paid for Hank's rehab, and I still believe that will ultimately play out. The turn of events which cause that to happen, I am not sure, but I don't think the finding of the book in the bathroom will be what does it. I somehow think Walt will be able to explain that away, or maybe Hank doesn't confront Walt, and continues to investigate. Eventually, something will require Hank being told what's up.. my guess is, some unbeknownst German/Czech bad guys show up, and Walt's family is in real danger.
The other element that I have believed for a long time, will play a key role, is Walt Jr. (Flynn). I think he is somehow going to be killed, possibly with the rest of the family, but I believe he will somehow be important to the details, he will inadvertently lead hit men to the house, or befriend one of them, or something. He may find the Ricin and think it's drugs, taking it...then Walt has to watch his son slowly die, not able to save him. Something is going to take Walt's kids from him, this is his 'most valued possession' and it's somehow going to play a part in his downfall.
It's going to be interesting to see what happens now, with him "being done" and all... I don't think he will be able to just walk away. There are FAR too many lose ends. Jesse with 'fat stacks' will be interesting as well, and his reaction to the money... that is interesting. I somehow get the feeling Jesse is very conflicted morally, he sees that money as 'dirty' and can't live with a clean conscience about it. Don't know what happens there, but I have also thought it might be possible for he and Walt Jr. to cross paths, and maybe that money plays a role in Jr. becoming involved in the whole thing? Lots of questions remained to be answered and anything can happen with this show.
Correction on earlier comments, or more of a point of order, Season 5 is the final season of Breaking Bad. The last episodes are scheduled for 2013.