This fic is eye-catching, and beautifully written, but I have to ask: why are you bothering with TF2 in the first place? These characters don't seem to have much to do with the game or the characters. If you were to go through the fic and delete all instances of "Medic" and "Heavy", or "RED" and "BLU", this would still make perfect sense, because the characters are different enough from the canon classes to work wonderfully on their own. They don't need to be tied down into being TF2 OCs. Your characterization is better than that; one would need no prior knowledge of the Medic/Heavy classes to fully understand this story and the characters within. In my eyes, this isn't really a TF2 fic. It's great, but not TF2.