We don't know exactly what Turtle Rock Studios is working on, however, they have listed an available position for a senior level designer. And TRS is very picky. They are looking for someone who has previously worked with modern FPS engines, and wants someone who has already worked on a competitive FPS game. Whoever it hires will create "competitive and balanced map layouts".
But wait, there is more! They also need an environment artist.
It does not say what they are exactly working on. However, 2 years ago, the dev team said that they are working on a dark fantasy co-op FPS—a "new franchise set in an all new universe". And that is all we know.
So what do you think it will be?