This essay covers slightly different ground. Like her, I believe that both Snape and Pettigrew are goners, but instead of the chances of their deaths in book 7, I'm going to describe the manner in which I believe Snape and Pettigrew will meet their ends, and describe a literary motif that unifies both their demises.
The motif is the idea of Ragnarok - or another way to describe it might be M.A.D.: Mutually Assured Destruction. The tale of Ragnarok depicts several fated pairs of gods and evil beings who are ancient foes: each god is uniquely equipped to kill each foe, and in the final battle both members of each fated pair die in the process of killing the other.
Now, we know that JKR has read her Norse mythology: the very name of Fenrir Greyback tells us as much. I'll describe how I believe she will apply this Ragnarok/M.A.D. motif to the death of each character in turn.
Peter Pettigrew and his Silver Hand
All the revelations of past books have convinced me that JKR has a whole basketful more easter eggs to give us in Book 7. As far as I'm concerned, Pettigrew's silver hand is one of those plot bombs waiting to go off. Ever since Book 4, I've been asking myself: "Why silver?" The symbolism of silver (light, purity, the moon) does not really make it a suitable 'gift' from the Dark Lord: an iron fist would have been far more symbolically suited. So, if not for the symbolism, then why?
To be sure, other people before me have speculated that Pettigrew would kill Lupin with it, but to me that never quite made sense. After Book 3, Pettigrew had a Life Debt to Harry: murdering Lupin would be a damned poor way to pay it off. It wasn't until Book 6 and the sudden introduction of a second werewolf, and what's more, a thoroughly evil werewolf, that Pettigrew's hand made perfect sense. Greyback, not Lupin, is the foe Pettigrew is uniquely equipped to kill.
I suspect that at one point in the final battle, Lupin and Greyback will engage in a nicely symbolic combat, mano a mano, (or possibly lupo a lupo), but will be so evenly matched that they'll fight each other to a standstill. Greyback will order Pettigrew to finish Lupin, but instead Pettigrew will turn on Greyback at the last possible moment and kill him instead with his silver hand (possibly by ripping his jaws apart, since that is how the Fenrir of Norse mythology died at Ragnarok). However, Pettigrew too will die of the wounds Greyback gives him in the process. By saving Harry's sole remaining father figure, Lupin, Pettigrew will have repaid his Life Debt to Harry, and given the readers a moral lesson in the value of mercy.
Now, moving on from Pettigrew to a character much closer to my own heart...
Severus Snape and his Phoenix Patronus
On her site, JKR had a lot of interesting things to say about Patronuses, including: "each Patronus is unique and distinctive, so that there is never any doubt which Order member has sent it; nobody else can conjure another person's Patronus, so there is no danger of false messages being passed between Order members"
If, as I have already argued elsewhere, Snape is a deep cover agent still spying on Voldemort, then he has four possible missions. The first two, assassination and sabotage, are missions more commonly carried out by deep cover agents, since they do not require communication between the agent and his true side. The last two types of mission are misinformation (feeding the people one is spying on false information about one's true side) and intelligence gathering (which is what most laypeople think of as 'spying').
Snape, at the end of Book 6, has a cover as deep as any agent could possibly wish for. However, simple assassination is out: I find it impossible to believe that Snape does not know about Voldemort's horcruxes. Sabotage is possible, but beside the point: no mission other than the final death of Voldemort would be worth the price Dumbledore paid to secure Snape's cover. Therefore, we're left with intelligence gathering: Snape must be trying to find out where Voldemort's horcruxes are, so he can either destroy them himself, or direct others (Order members) to destroy them. But intelligence gathering is one of the missions that absolutely requires an open channel of communication between the agent and his side.
How could Snape possibly get a message out to the Order members? I firmly believe that the only device that could serve that purpose is a Patronus - which as JKR tells us " is an immensely efficient messenger for several reasons: it is an anti-Dark Arts device, which makes it highly resilient to interference from Dark wizards; it is not hindered by physical barriers [...] nothing conspicuous needs to be carried by the Order member to create a Patronus."
But Snape's problem is even worse than that: how could he ensure that his message is believed and trusted by the Order members? Not just any Patronus will do: certainly not Snape's own Patronus, which the other Order members would recognise and distrust. From Tonks' Patronus in Book 6, we know that Patronuses can change their shape in response to a great emotional upheaval. And from JKR's note, we know that "Dumbledore's Patronus is indeed a phoenix." If Snape's Patronus has changed form to Dumbledore's phoenix, surely that would be a sign potent enough to give his messages the credibility they need.
So, in book 7 Snape will have a new and unusually powerful Patronus. And, like Pettigrew, Snape carries a Life Debt - to Harry's father, which was never discharged - and an added burden of guilt over Harry's parents' deaths. What is the foe Snape is uniquely equipped to kill?
Remember that all through Book 6, ominous descriptions are given of widespread fogs that are a mere side-effect of dementors breeding. Voldemort is raising an army of dementors. Remember that the Patronus is described as a very difficult spell: not all wizards can cast one at all, and fewer still can cast the fully-corporeal Patronus that is capable of driving off a single dementor. Remember, also, that because of the sorrows in Harry's past (including the same deaths of his parents of which Snape is at least partially guilty), Harry is uniquely susceptible to the effects of dementors: he was the only one on the train to Hogwarts to faint from his exposure to them.
I envisage a climactic scene where Harry is surrounded by a limitless sea of dementors: what situation could be more hopeless? Until a phoenix Patronus blazes into view, driving them away, burning them, destroying them... a Patronus that, Harry sees as the last dementor disappears, was cast by Snape. I fear that it would take a scene as dramatic as this to convince Harry, however belatedly, that Snape was working all along for the right side. I also fear that, whatever the details - whether Snape has to throw his last energies into the Patronus, until he collapses, drained of magic and of life, or whether he has the ill fortune to find himself surrounded by that sea of dementors, and is destroyed by the last one as he is destroying it - he will not survive the end of Book 7.
My only consolation is that, if and when that happens, he will be posthumously regarded as a tragic hero: by fandom as well as by Harry and the rest of the wizarding world he died to save.
1 Inferi are another possible candidate, since Voldemort also has an army (or at least a lakeful) of them, and they are afraid of and driven back by fire; but in my view they are much more unlikely. The Patronus is the only spell known to affect dementors, and thus far better fits the requirement of uniqueness, whereas it seems likely that any form of fire would suffice to repel Inferi.