I talked about it more in depth here:
tldr: Valk covers way, way too many skills deficiencies of Mercy’s already shallow skill pool, It’s too forgiving and lacks room for player expression, it’s a design issue, not a balance one.
Among other things, I would argue for the complete removal of chain beam, the infinite mobility and self heal boost, give those just as powerful alternatives that require more skill to use.
Have a single target beam that is more powerful, rather than chain beam that removes target choice.
Have GA during Valk become enhanced, but don’t completely negate it’s necessity with flight that only covers for bad positioning.
Have Valk give a couple of immunity frames during activation rather than insane self heal.
Let Valk dual hand battle Mercy rather than giving infinite ammo and projectile speed that cover for aim.
There’s endless things I could suggest honestly with the point of making Valk better.