I saw a suggestion about this somewhere on the forums, but I think this would be a really good idea. As it is right now Mercys pistol uses a projectile which is unreliable at long distance, and can be somewhat difficult to be consistent with even in close quarters fights.
Mercy with a hitscan pistol will make her be able to farm her ultimate when given thre opportunities to do so. This will also make her more consistent at defending herself, as it currently is most heroes land bust damage to get a one shot kill or a near one shot kill making it hard for a Mercy to actually defend her self from something like that.
Overall, I think Pistol needs to be more than a meme ability/utility for the super aggressive battle Mercys like me.