In many games in general, from FPS to RTS, there is one component added to enhance combat efficiency (at a price) and therefore widen the boundaries of playing strategy; the presence of and ability to use some type of stimulant to boost speed and attacking power (think the 'stimpacks' of the Fallout series and Starcraft 2). It would be a good idea, in my opinion, to implement this element in the game with some balancing factors such as exclusive availability to the Medic for use and distribution, and negative side-effects such as injury as a punitive measure against abuse.
Alternately, one could consider temporary invulnerability that could be dispensed by the Medic yet again, a la the infamous 'Ubercharge' of Team Fortress 2 that can be deployed by a Medic onto a player with devastating results for the enemy (particularly if the invulnerable player is capable of high-damage output!).

we already have this. Each class can "boost" a trait of the other players on their team.
Medics boost health
Soldiers boost ammo
Engineers boost weapons damage

I think that this would destroy the class balance. All classes have a purpose and have certain abilities. If one class has to give other the stimpack, you'll change the class balance. Besides, like Jordan said: We already have health buffs and weapon damage buffs. (I don't consider giving ammo a buff) Then there's the SMART system which gives lights more speed and the ability to reach more places. With a stimpack all these carefull considered balances would need a shift or a rebalance.

Invulnerability gives one player too much advantage in this game. Imagine an engineer doing an objective being invulnerable, he could easily complete it even when there are 8 people shooting him.

We already discussed something similar to this in the Medic discussion. No invulernability to OP. For the same reasons the operative can;t go invisible.

as much as i love starcraft, no

I thought you were refering to a skin of the model for syringes to be like fallout's stimpacks. If you don't know what they look like, here's a picture
http://images1.wikia.nocookie.net/__cb20090504141456/fallout/images/2/20/FO3_Stimpak.png
This would actually be pretty cool for several reasons
1. Looks kinda like a syringe Resistance might have
2. It's from Fallout, a Bethesda game, so it actually has a chance of being in the game
3. It would just be a nice easter egg

And Fallout's stimpacks are just for healing purposes, not boosting stats, unless you want the resistance to start taking jet...