My opinions and suggestions for Mercy, give it a quick read 
IMO the current state of Mercy feels very boring and it makes me feel useless whenever I play Mercy. And also to me, she seems a bit unbalanced as compared to the rest of the Support Heroes. So here is what I think can be done to make her balanced and fun again.
But before that, I want to say that she is not balanced because all of the Healers have some sort of way to negate an enemy ult.
Ana: Sleep dart, Nano Blade can kill a 200hp target while in Zenyatta Ult if the Genji pulls off a Slash and Swift Strike consecutively.
Zenyatta: Rapid healing in Trancendence.
Lucio: Damage blocking from Sound Barrier.
Previous versions of reworked Mercy: Undoing kills by resurrecting.
But now Mercy’s ult only gives her more mobility and chain heals and boost. Although I agree resurrect overall is a very tricky ability as it is extremely game changing so I think rez is in a playable state right now. Although I am keen for ideas that I’ve heard from others such as earning rez (from EeveeA_) in ways like Torb collects scrap, Mercy can damage boost. Because Valkyrie (in previous versions of Mercy) was like 2 ults in one, as it was the ability to solo rez anyone anywhere, and keep everyone alive and the Mercy was hard to kill for the 20s of Valkyrie. That said, I will move on to my suggestions.
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Valkyrie
Upon activation, maybe it could grant the Mercy herself and her teammates, NOT invincibility, but rather, the ability to avoid death. (Hence, this would be super appropriate to the rerun of her voice line “Heroes never die”) Granting her teammates invincibility would be too overpowered because as compared to her ultimate Resurrect, the teammates were immobilised when invincible due to the fact that they were being resurrected. So granting invincibility upon Valkyrie would give them the ability to deal damage carelessly, and adding side effects such as disabling them from dealing damage would be too complicated, so I think they should have the ability to avoid death for 2s, but can still be damaged to critical health. (25% health)Also I think she should be able to do maybe 1.2x healing overall to her target selected, and chained teammates when healing in Valkyrie, OR maybe her selected target could receive 1.5x more healing whilst chained teammate receive normal 1x healing. I’m only suggesting this as her E ability has been nerfed to a 2s cast time that comes around every 30seconds, thus giving her less potential ult negation. And maybe even that’s still fine, but now she is easier to kill with the 50% Guardian Angel speed reduction, and Valkyrie brought down to 15s, so I think these changes are reasonable and will balance Mercy a lot more.
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Guardian Angel
I think that the initial movement mechanics overall from the first release of Mercy 2.0 into live servers should be brought back. Although it was addressed as a bug, the knowledge of how her movement worked required to successfully take control of extending your GA travel distance was something that the better Mercy players could, not exploit, but rather take advantage of, and honestly, Angel Bouncing (or Bunny Hopping) with this version of her movement mechanics made her extremely fun alone. I do like the current way her movement works, being able to gain air time from using GA, but I’ve realised that GA feels like it takes much longer to come off cool down this way.
These are just ways I’ve come up with and took inspiration from all the controversial threads and videos I’ve seen in the past that have accumulated in my head.