In terms of Balance it is the most optimal thing for her. Not to strong, flows with her kit, gives her more midfight potential and raises her skill ceiling a lot.
And the main point of Pacify is to be a more active skill to make her more attractive for other players.
I was thinking about a good E ability for her a lot and that was the only thing which really fits and would work.
About other ability types:
If we give her a stun, she would be again frustrating to play against. Most other ideas sound good but they always conflict with Mercy’s weaknesses.