I have several things lined up to blog about that aren't TF2, but screw you.
It looks like the Spy is going to be the next update, what with the TF2 blog having an update with no actual information, but with the rather obvious title of "Gentlemen," and the Heavy Achievements include "Spyalectical Materialism", which features an image of the spy wielding a silenced pistol. There's also my theory that each update "cancels out" the previous update, (Medic -> Pyro -> Heavy -> Spy could be a "Rock Paper Scissors" style classification of those classes.
Also, they have always updated the classes that see little play, and Spy is currently #2 least played character. Personally, I have no idea why this is, as Spies are simpy ridiculously awesome on Badwater Basin. Whenever I'm playing without one, I feel the need to immediately switch.
Anyway, my backup, "wacky" theory is that they update the Sniper and Scout simultaneously. A valve person has pointed out that they definitely don't want to do the Sniper update all by itself, as playing a team of 90% Snipers would be absurd. (Unless it's 2Fort, where it's all too common.) Scouts are the least played class, and I'd really like to see them made more viable in a world where Engineers are the most played class and Sentries are all over the place.
My backup less wacky theory is that they do Sniper and Spy simultaneously.