There are many, very good suggestions out there to give Mercy Mass Res in a fair way, which also allows an E ability. That would be enough to make her viable in Pro play, while not making her OP and still fun for the normal community.
Well… We made a Mercy over the duration of a month with more than 40 players from all ranks (S-T500) and many different heroes.
Under “A Solution” you can find it.
This idea is completely rebuilding the way how Resurrect as an Ult works and adds a new E ability which fits very well into Mercy’s overall design.
So far the idea is very well received. So that would be a very well, collectively created, balanced and discussed way to rework her.