I came up with a way to make Mercy more versatile and have more flexibility and protectiveness.
Resurrect:
Cast time increased from 1.75 seconds to 2 seconds
Can cancel at any point by hitting resurrect bind, cancellation causes 15 second cooldown
Valkyrie:
Grants additional resurrect charge upon activation
Resurrect cast time decreased to 1 second during duration of ult
Movement speed decreased by 35%
Let me know what you think.