I've gone on and on about the cyclical nature of the show. From Walt himself, the way he's reliving a much more violent, destructive version of his earlier life, to the meth business itself. Everyone involved seems to follow a pattern of events and consequences.
You only have to barely think about it to see that Gus and Walt's lives are nearly mirror images of each other. The incidentals play out differently, but each one ends up finding themselves in strikingly similar circumstances.
Okay, this is gonna be kinda long, so bear with me...
Think about Gus' life in the drug business. He and his friend want to start manufacturing meth. There was money to be made, and they wanted their share. They get together, cook up a bit, sell it around town, then go to the cartel for their blessing. They're greeted with open, friendly arms, and it seems to be the start of a nice working relationship.
...until Gus' friend gets a bullet in the head for their effort. He's executed right on the spot at a supposedly friendly gathering. Gus is spared due to who he knows. The cartel doesn't like him, but they can't touch him. Gus, of course, feels betrayed.
Despite all these hurt feelings, Gus continues to work for them, brooding and seething all the while. The cartel believes they have him under control. They've think they've made their point, and assume he'll stay in line. Gus goes north of the border, and builds their business into a thriving empire.
Once Walt enters the picture, Gus realizes he's amassed enough power and leverage to both strike out on his own, and take his long delayed vengeance against those who wronged him all those years ago.
...and I'm not gonna go into the details, but you all know how it ends. The cartel can't make a move against him, and Gus knows it. All they can do is make examples of the people around him. He's openly defiant, walking into a hail of sniper fire because he knows...he knows...they can't pull the trigger on him. He's too important to their business. It all ends with him being welcomed into the home of his most hated enemy as a show of respect, and it's there where he makes his final move.
Now contrast all that to Walt's life in the drug biz. He and Jesse want to manufacture meth. He's dying, and he wants to leave a nice nest egg for his family. They bake up a few batches, sell it around town, a bunch of misadventures happen, and Walt eventually meets Gus.
They get together as businessmen, sharing a mutual respect for one another. Long story short, Gus very soon finds out Walt's too unstable to rely upon, and needs to be rid of him as soon as possible. Thing is, he can't do it. Walt's too important to his plans against the cartel. Gus attempts to finagle a replacement, who Walt quickly dispatches, leaving him without any other options. He has to endure Walt. He can't kill him because of what he knows.
And so even more longer story short, Walt kills Gus, and creates his own empire. He meets Todd, who introduces him to the nazi bikers, and things go swimmingly. Walt's sitting on 80 million dollars, and decides to retire from the business. It all seems to work out perfectly for him...
...until Hank finds out, which leads to him dying in the desert, which eventually spurs Walt on to revenge.
In the meantime, Lydia is working with Todd, trying to fill the void Walt left when he retired from the business.
So if you look at it like that, Todd and the nazis are to Walt what Walt was to Gus. The wrench in the works, the fly in the ointment, the people who wronged him. So if I kind of build up from this, here's what I think is gonna happen...
-Walt will kill Lydia. I'm thinking with the Ricin tablet.
-Todd will realize this, and it'll piss him off. Greatly.
-At some point, Walt will end up walking into the nazi compound out in the desert, acting all buddy buddy before he strikes out at them, likely using explosives, that big gun, and all other kinds of cool stuff. Todd probably won't be at the compound during the attack. Rule of drama will probably have him discovering Lydia's body as it's all going down.
-But Todd will eventually kill Walt, picking up the reigns and continuing the meth trade using his formula.
Jesse? I dunno where fits in. He's one of many wild cards in play here at the end. He'll either live to provide that one silver lining to an otherwise brutal ending, or die as that final gutpunch to the audience.
...holy crap, this was a long post.