There have been multiple suggestions that deal with before people die, but we already know that bringing back someone from the dead does nothing to guarantee success, but saving your team from a wipe before they die often does more for a team’s chance of success.
I’d be more than willing to have one that would hold people on the brink of death for example, but it would still provide no counter play.
Mass Invulnerability, but still no counterplay.
You can’t give Mercy something as impactful as Moira’s ult even though they are in the same sort of support area, because it would be considered unfair because she is “too easy”.
You can’t give her something like Valkyrie because it does nothing to excite or make the player feel empowered because you are doing the same things you are already doing.
You can’t give her Resurrect because people have a complete and biased hatred toward it even though she was never OP in the lifetime of it and only ever reached a decent state when they added the invulnerability nobody wanted.
You can’t keep E Rez on her because it is still far too powerful and it is not fun to turn into a snail for a regular ability, no one else except Sym and Reaper are forced to struggle for regular abilities. For example, in a fast paced game like Overwatch, Sym and Reaper’s teleports should be fairly quick to make, it should take years and it shouldn’t make them as vulnerable and easy targets as it does make them because being still makes you an easy target which shouldn’t be considered fun to play or play against because no one should be okay with taking cheap kills especially when they have far superior kits to take advantage of. This is the same for Mercy and snail Rez.