It’s an ultimate ability it doesn’t have to be fair or fun to play against it just has to be at about the same level of unfairness/unfun to play against as the other heros.
I think I’d put it on a new character that is relatively similar.
From what I understand (I didn’t personally do it or see it much since I just play in QP), Mercy players that hid and res’d did so because they/their team felt the value of having Mercy in a fight vs having Mercy being out of the fight and then coming in and resing favored the latter.
So, I think I’d make the movement abilities somewhat less effective on the character to make it a bit harder to be away then dive in.
I’d make getting health back for the character require them to be actively doing stuff so that if they just hid after taking damage they would still be damaged.
I’d make it so the character had a somewhat more active kit so that choosing to stay out of an engagement would be more of a loss.
I’d alter the ultimate so that the ability would do more to help the living and less to help the dead. I’d keep the range and the lack of line of sight and get rid of the invulnerability.
I’d suggest something like the following where the ability has multiple stages:
Stage 1: Starts Instantly: 5s heal over time for 40/s on everyone that is living in the area.
Stage 2: Starts after .5s: Res everyone who is dead with 20% of their hit points that is in the area. (Healing the damaged part will not provide ultimate)
Stage 3: Starts after 3s: 750 hit point AOE heal/overheal spread evenly among the team that is in the area.
You’d need to play around with the numbers to ensure you got the results you want for various comps and you’d want to also avoid stepping on other ultimate abilities areas.
The goal is to make it so that you will still be able to res as many people as you like but, you’ll be trading getting more people up for getting less from the HOT and the people you’ll be getting up won’t be at full hp.
I’d also suggest making the character appear as an evil omnic that is not attractive. Mercy had Mass Res for a while and only really seemed to get lots of complaints when she became highly picked across the board. Being a pretty heroic human character likely inflates her pickrate. The omnic characters, ugly characters, and evil characters tend to be less popular which would make it less common and generate less complaints.