According to the latest PC Gamer, the next class to get unlockable items in Team Fortress 2 is the pyro. Two months ago Valve released the first class update for the medic, which received three new weapons and 36 achievements in April. Since the update the medic is now the 3rd most played class.

Not much is known about the future Pyro update, but I am sure you can expect some rediculous flame thrower action in the near future.