Okay so let's make this quick. Basically, Mercy is fucked up really bad. This is Blizzard's fault because resurrection is inherently impossible to balance in a game and Mercy not only had the only rezz, but also an instant-use AoE rezz. No shit this was going to be a problem. Now Blizzard is haphazardly trying to balance her rezz, attempting to make it "Good enough to use" without being "Overpowered", but still without being "So weak and nerfed it's useless".
Bottom line? She can't have a rezz. It's too hard to balance, so fuck it. Remove it.
What her ultimate should do is, when she uses Valkyrie, she becomes a floating invulnerable golden healy bitch who zooms around in the sky, healing people for a decent amount in a little AoE around her. The main draw of the ult, however, is that during it, she is able to lift up and carry a teammate. The teammate is also invulnerable during the duration of the ult/them being picked up by Mercy.
Why is this a better alternative to one of her rezz-based abilities (and whatever weird angle they're trying to go with with her current ult)? Because it makes Mercy reactive instead of responsive.
Before, Mercy just ulted whenever her team got wiped. This was very boring. Nothing was exciting about watching one team get wiped to an ult-push and then seeing Mercy rezz everyone. It also totally invalidated the enemy team's push. With my ult conception, now Mercy is forced to react to the enemy team instead of just responding to them. When she's on the front line with her DPS or tank and they hear IT'S HIGH NOON, now Mercy can pop her ult and invuln/carry off her teammate, instead of the McCree getting a kill only to have Mercy on-demand rezz his frag.
Edit: One issue with my idea is that if Mercy's teammate is also invuln while Mercy is carrying them, then instead of saving them from the frontline, Mercy can just ult them and sit there until the enemy's ults are used, then plop them back down in place. Or should could carry an ult-ready Reinhardt behind enemy lines so he can ult there (or maybe that'd be an intended use? I dunno).
A fix to this problem might be that the person she picks up is only invuln for 1-2 seconds. Enough time for her to fly them off, but she can't keep them in enemy targets or else they'll kill the teammate. She has to carry them off to safety.
Ok there you go thanks.