Team Fortress 2 is getting a substantial new update in the form of the Gun Mettle Campaign. The campaign will offer two contracts per week over a three month period, with each offering a new skill-based challenge. Examples include "get a kill with a reflected projectile as Pyro", or "survive 1000 damage in a single life as Heavy".
Completing contracts will grant campaign-exclusive weapons or an unlockable weapon case. These weapons will come in six grades of rarity but won't give you a competitive advantage, according to the Gun Mettle Campaign FAQ. Instead, each (and by 'each', I mean every single weapon assigned to every single player) will come with a unique paint job. If you don't like a weapon, you can sell it.
Access to the Gun Mettle Campaign will set you back $5.99. Some of the profits will go to the map creators responsible for maps featured in the contracts. These maps, which come in the form of Borneo, Suijin and Snowplow, as well as the new Valve-built map Powerhouse, will roll out free for anyone not partaking in the Gun Mettle Campaign. More info over here.