Team Fortress 2 may have another mode incoming, one that pits the red and blue teams against another team of a... mechanical variety.
Numerous hints beginning as early as 2010(!), which have been compiled in one convenient Reddit thread, make a strong case for a future mode that involves fighting off waves of robots. "Mann vs. Machine," the supposed update is rumored to go by.

Users caught on to this thanks to a splatter mark that recently appeared on the logo of the Team Fortress website. It links to a picture of a birth certificate, one that mentions the two brother who founded the competing teams had a third sibling, Gray, who, not long after birth, was "absconed with."
The assets hidden both within the game and its website hint and detail robotic versions of the iconic classes. Presumably, these metalic varients will make up the ranks of Gray's team. Reddit posters think references to deployable items the engineer can use indicate tower defense will play a role.
Callum Rakestraw, NoobFeed.