Too much politics! It's game time!
As I noted in the Pyro entry down below, Valve adds special weapons for characters when it gives a big update to TF2. These special weapons are geared towards a particular playstyle, and have advantages that endear it to that role and weaknesses that make it unpalatable to other players with the same class, different role.
With that in mind, here's some ideas for classes that Valve hasn't updated yet.
Rocket-propelled Rocket: For the Soldier
Normally a Soldier carries a four-shot rocket launcher (4 loaded, 20 reserve) designed to engage groups of enemies with splash damage at short to medium range. The rockets are slow-travelling, limiting their use at longer ranges.
The Rocket-Propelled Rocket is designed slightly differently. It's a much larger, much faster moving rocket that does more damage and has a greater area of effect than the normal rockets. Downside? You only get one shot before you need to reload, and the animation is significantly longer than reloading normal rockets. I'm thinking 1 loaded, 10 reserve, but that's subject to balance issues.
So if the RPR carrying soldier is engaged at close range by a normal soldier, the normal one would probably win. If the RPR soldier catches the normal soldier with his buddies off guard, he can fire his rocket to massively weaken or kill a large group of targets.
Semi-Automated Pistol: For the Engineer
The most common tactic for Engineers in Team Fortress 2 is simple--hide behind your sentry nest. If the sentry nest refuses to take something out, like a disguised spy, the Engineer can engage with his shotgun at close range or with his pistol (12 loaded, 200(!) reserve) at longer ranges.
The Semi-Automated Pistol (I'm thinking 12 loaded, 120 reserve) is designed to encourage the nesting habit. At any time, the engineer can set it down; it will quickly restructure itself into a level 1 Sentry gun. He can use this in addition to his normal sentry to cover a larger area, or use this new sentry to cover his back. He can repair and reload it like his normal sentry (hit it with his wrench), but he cannot upgrade it.
Should the Semi-Automated Pistol be destroyed as a sentry, that's it; the Engineer cannot reacquire it until he dies and respawns. Further, the pistol-to-sentry transition is one-way; he cannot recover the pistol, leaving him only his shotgun to engage targets not covered by his sentry.
Team Fortress 2 players, what do you think?