It’s true that Mercy’s original kit had problems. As much as many other Mercies will hate me for saying this… Mercy actually did need a rework. Just not the one she got.
Mass Rez itself needed a rework. It didn’t have enough proactive counterplay, while at the same time not being proactive enough itself.
Mercy’s base kit needed rounded out. She needed a midfight E so she wasn’t so appalingly ult-dependent, and the GA bunnyhop was that tiny bit of independent mobility that Mercy players craved
Though also having a glide instead of a featherfall passive wouldn’t hurt. Being able to commute between platforms on your own would be a big QoL upgrade.