TF2's Christmas update is a Big Rock one. The new Big Rock map will contain the Mecha-Engineer, expanding the roster for the Mann Vs Machine mode. Valve explain: "These Grinch-like Smissmas cakewalk destroyers can skip huge areas of the path with their teleporting ability, and also teleport other robots forward instantly with their "telling other robots about the teleporter" ability."
There will also be new weapons. But you knew that. Right? There's also a new comic. Hooray for holidays!