TF2 isn't even done with content yet, it seems kind of pointless. And if Valve is still using Source, its also kinda pointless, unless they just made new mechanics with slightly different graphics. Which wouldn't be TF likely after the nice image Valve has gotten the game.
So basically, no until they get a new engine. That entire expansion vs new game idea applies here.
So basically, no until they get a new engine. That entire expansion vs new game idea applies here.