I still hate playing her, but I play her once in a while, and follow the same priorities as I did when she was 1.0.
My main thing is to heal. Always heal. Keep everyone topped off. IF someone has an ult ready, and your damage boost can assist them, do it. Or, if you're ulting now in this new mercy form, alternate between healing and boosting. If an enemy is rather close to you, take them out with that pistol. There are some pretty good plays I seen with a DPS Mercy, but sometimes that is a gamble.
Also, when not ulting, if you have a Zarya and she is ulting, and has the enemies trapped, join in on the shooting, or boost your teammates.
Resurrections are different now, obviously. Rez when you can, IF it is safe. Getting yourself killed by rezzing when the enemy team is in plain sight is not helping anyone. Save it if you need to for crucial moments. When ulting, and you got multiple teammates down, rez your best dps and your tank if you have one. They can help you keep that points/payload until the rest come.
I typically don't play her anymore, except if a friend is getting tired playing healer, in which case, I want to give them a break to do new things too. I been following those priorities for a while now, and they really help us out. I used to main her a lot, but now, I like my Junkrat and sometimes my Reaper.