Zen definitely needs a buff, he’s almost never picked because 30/sec just isn’t enough on top of how easy he is to kill. I think making his pocket 50/sec would be enough to make him viable, no other changes. With all the heal nerfs that came through, it’s pretty rare that it’s a good idea to pick a support that heals so little, because your other support can’t take up as much of the slack as they could before.
The way the game is playing after all the heals nerfs that came through a few months ago, you need to be dealing damage as a support in addition to keeping your team up, and Mercy doesn’t do that. It seems broken that she heals less than Moira, who can also clean up kills at the same time, and who also has a really good escape tool. And her damage boost is less good than Zen’s discord orb, which effectively buffs your whole team against a particular (and easily switchable) enemy. Basically everything she can do except res can be done better by another support. I think buffing her beam to 70/sec would be enough by itself, or else her damage boost to 50% so it’s actually worth (If you’re a good shot it’s currently better in most cases to whip out your pistol than to damage boost a teammate.)