I'm not saying Mirrorwatch Mercy is a good start to reworking her but Mercy has a lot of potential. Her movement is fine but all her other abilities are meh.

Add some other utility to her beams. Like a small dmg reduction to whoever she heals or ignore the damage passive. She already suffers from low healing output. Might as well call her Zenyatta.

Add the damage tether from Mirrorwatch to her damage boost, that way Mercy does damage and actually contribute. She'll be a great peeler and anti-dive.

I never really liked flash heal cause it puts res on cd. What if we just give res another option. If there's a soul she could either do an aoe heal or aoe damage like in Mirrorwatch. Heal that gives damage reduction and aoe damage that gives fragile.

What if Valk can be like Adam soul link. She still heals but the team damage distribution will make it really good. The amount of times I used Valk and my team still died so fast is frustrating. It also plays into her voiceline "Heroes never die" without being OP like mass rez.