Coming up with alternatives is quite fun, though I'd quite like them to keep her similar to how she is, as I find the rez unique.
If they had to change it, my idea would be:
Guardian Angel (totally stolen from WoW)
Press E whilst healing someone, and if that person hits 0 between the time you pressed the button and, say 5 seconds, they're saved and after a brief stun (0.5s) brought back to full health. They glow brightly to indicate to the enemy they will do this, so the enemy can try and hold off killing you, or be punished for not doing so and trigger the action.
I also think Mercy should be allowed to overheal by 25% during her ult, to make up for it not being as awesome, which forms a shield if the player being healed is full health, which slowly fades if Mercy changes target or the ult finishes, whichever comes first.