I found this thread on the Steam Forums by a user named “slugz“. He sent an email to the Valve team asking if new achievements will be introduced in the near future. Erik Johnson (Valve Project Manager) replied with:

Thanks for the mail Thomas.

We’re planning on adding achievements to all of our games you touched on in your mail. We’re just now coming out of the post-shipping haze, so these should start showing up on Steam pretty quick here.

We’re also planning on adding a bunch more to TF2, starting with an achievement pack for the Medic in the next week or so.

Erik

As you can see, we should be looking at a whole bunch of new achievements to use and conquer in the near future. We should be seeing these achievements for the Medic class in the very near future (a-week-or-so). I look forward to these as like many others and I will keep you all updated.