PC Gamer blog has shed some light on the changes Valve has planned for Team Fortress 2. The "large scale modification" is actually quite big -- new weapons unlocked via achievements earned in the game. The arsenal can be swapped out via a new Loadout menu, though we're not sure if that means configuration can be done mid-game, between rounds, or only from the main menu.
As previously stated, the medic will be first to receive the upgrades. The first earned weapons is the Overhealer. Unlocked by earning 35 achievements, the "gun" can permanently raise an ally's health to 200% (the current medic tool temporarily boosts health to 150%). Citing balance concerns, Valve said they may remove the invulnerability-granting Ubercharge from the Overhealer. The second unlockable weapon is also a healing ray (what? No shotgun?), but no details were given except that you have to earn all medic class achievements to unlock it.
A new game mode will be introduced with the map Goldrush, the second map due out (alongside the Badlands remake). In Goldrush, the blue team must escort a mining cart through tunnels. The cart moves faster with more blue team members nearby, but stops completely if a red player gets near. According to previous statements from Valve, the maps and medic updates are due out within the next two months.
[Image credit; thanks, Fortyseven]