I’m not saying she needs to be a better healer than Ana, but she should be more engaging to play and not feel like a left-click healbot. I think something simple might be:
-In addition to its other benefits, targets of the Caduceus Staff are immune to status effects (cannot be stunned, frozen/slowed, poisoned, hacked, discorded, slept, anti-healed). Does not apply to targets already under these effects (can only be used proactively).
This would slightly buff Mercy without increasing her healing amount, and allow good Mercy players to negate some of the above abilities with quick timing and foresight.