The idea is to just completely get rid of damage boost, and replace her alternate fire with the pistol. No weapon swapping, and no screwing up break points for other characters. Maybe she could even shoot while healing simultaneously. Obviously they would have to tweak her animations and stat numbers.
Mercy is literally fine. As long hitscan dps’s stay balanced she’ll be too. Mercy only starts getting problematic when you pair her with other overtuned characters. (Season 1-2 Sojorn, or release Ashe) for example