The cat has been let out of the bag, and thanks to PC Gamer we now know that these large scale Team Fortress 2 gameplay changes are indeed large scale.
Valve have decided that in light of the monotonous and straight-forward attacking affair that is turning TF2 into "Ubercharge Fortress," instead of nerfing the ability, they are going to be gradually implementing unlockable weapons for every class, starting with the medic.
The first unlockable for the medic will be the Overhealer. The current Medigun enables a 50% higher health total while buffed, but the Overhealer will permanently boost the healed target to double their standard health. With that in mind, Valve suggests that you will more than likely not be able to build Ubercharge while using it.
You won't be able to switch between the Overhealer and standard Medigun mid-life either , you'll have to select which you want from a load-out screen prior to spawn.
Details on the second medic unlockable aren't as clear, however...
Quote from PC Gamer:
it'll be another Medic healing ray, and it'll change the way the class plays all over again.
The first is earned by acquiring half of the 36 new Medic achievements, and we assume the second is achieved by acquiring the other half.
The two biggest concerns right now is that first of all, we're all a little worried that public servers will be a 12v12 medic affair as people rush to unlock these new weapons. If Valve were wiser, and released all of the new unlockables and achievements for all classes at the same time, this would be easily avoided and we could continue on with pub teams full of mixed classes.
The second is clan play, and the hope that a server side command is created which enables the unlocking of all weapons for matches. Otherwise, certain players and subsequently certain teams may have advantages over their opposing team, and clan play is optimistically meant to be about a level playing field.