Clearly the new Mercy is a problem and the rework was the main problem that caused it. Resurrect cannot work as an ability and has to be her ult so this is what I think should be done. She has to get an ability specific for her staff and one for her pistol.
Caduceus Staff - Healing: Mercy’s healing now builds up 15/30/60 or 20/40/60. This would build up only when she is actually healing someone and not if they are one them at full health. It takes 1 second to fully charge. This would stop people boosting because Mercy cannot always heal her Pharah and when she is low on health it would take a bit of time.
Caduceus Staff - Damage: This would work the same as healing. It would up 10%/20%/30%. It takes the ally to damage to charge up fully. But it works a bit differently than you would think. It starts at 10% and then after someone shoots someone it goes up to 20% and then has 1.5 seconds to to 50 damage to get to 30% otherwise it goes down.
Caduceus Pistol: This stays the same.
Guardian Angel: Stays the same, with the angel hop too.
New Abiltiy - Miracle Worker (could be called Valkyrie): Mercy gets a burst healing. She would do a burst healing for 120hp and then proceeds to chain heal her team but the person she is on gets 60 hp/s and her team gets 30. That would last 3 seconds. This would allow her to gain ult charge faster due to the nerf on her healing. It is on a 12 second cooldown.
New Ability - Defences Engaged: Mercy’s rate of fire is increased. She also takes 25% less damage and resets all cooldown. It lasts 2 seconds due to how much damage she can do. It has a 10 second cooldown.
Ultimate - Ressurect: This is the mass res from before the rework.
This would make Mercy’s primary kit more skilful, and would make Mercy players think about who to heal, when’s to heal and then when you get your ult it would be satisfying.