With all this talk about nerfing Mercy I started to consider what parts of her kit you realistically can nerf, and this is what I concluded:
Valkyrie suit (passives)
The only realistic nerf is to increase her healing after damage cooldown from 1 second to 2 (she used to have 3 but that made her unviable). I know there are people that want to nerf her glide to combat PharMercy, but I’m not even sure how that could be done without making it a hassle to use. I suppose in theory you could limit the amount of time you can use it in one go without using GA, but it really seems like it would just give the player a headache.
Caduceus staff (healing)
I’ve seen people demand she get her 50 hp/s back, but I’m not sure how that would even really affect what they want it to affect. It was changed to 60 hp/s for a good reason, and seeing as she has no means of increasing it I would say no to this. It would also be a nerf to her ultimate
Guardian Angel
At most I can see them increasing the cooldown slightly from 1,5 to either 1,75 or 2 seconds, but more than that will not work as Mercy needs it to perform her job as a mid- to close range healer. Some people suggest up to 5 seconds, which is ludicrous given Moira has the same cooldown for an ability she can use independently, with full mobility, and immortality and damage resistance.
Damage boost
It’s been the same since launch, and now because she makes Hanzo stronger it’s apparently an issue. It’s funny because it basically works like a discord orb, only instead of everyone doing more damage to one person it’s one person who does more damage to everyone they hit. If you also look at the average damage amplified the numbers really aren’t that impressive, so I don’t think it’s that particularly strong outside of ultimates of boosting particularly skilled players (on average 700 dmg is amplified).
Rez
How could you possibly nerf this more without it becoming a laughing stock of an ability? A longer cooldown would make it seem more of an ultimate than it already does, and forcing someone to stay still even longer should never happen for an ability that doesn’t make you invulnerable (like Mei’s Ice Block). If they nerf the slow even more it reduces what little skill you can show by knowing when to use walls to strafe behind. So really, you can’t tocuh this anymore in its current state. I guess they can remove it/replace it, but that would be the ultimate sign of failure to balance it on a cooldown and would make many in game assets and merchandise redundant.
Valkyrie (ultimate)
In theory they can make it so the chained beams heal/boost less, but the ultimate doesn’t have to feel more flat than it often does already. Her flight speed has been nerfed already so I don’t see them touching this either. It wouldn’t surprise me if the uninterrupted regeneration was either nerfed or removed completely, but it would just force players to hide even more above roofs and walls (which doesn’t make for very fun gameplay).
In conclusion
There really isn’t that much left to pick away from her unless you want to truly nerf the core of her character. I think the best, if any nerfs are to be done, is to nerf her regeneration cooldown from 1 second to 1,5 - it would force us to play more carefully, but all in all it wouldn’t affect the fun aspects of her character (like GA) as much and it would make it easier for the enemies to punish Mercy players out of position.