DON’T use any of the old versions of mass rez. Instead:
AoE rez in 12 meters (same as Lucio’s new aura)
.8 second cast time (for reference, the stun duration on Mcree’s flashbang is .7 seconds). If Mercy gets stunned or killed during the cast, rez doesn’t go through and Mercy loses all of her ult charge.
Line of Sight required. No casting through walls or around corners.
No invulnerability. Rez instead releases a 250hp burst heal to all still-living allies (and Mercy) after she finishes casting rez. This shifts rez to be a midfight ult, like Trance or Sound Barrier. Mercy would still need at least one corpse to use rez.
If Mercy uses this version of rez too late in the fight (e.g. hide n rez), she’s more likely to die during casting than actually get off a successful rez. The burst heal makes it much more profitable to stay with the team instead of slinking away before you use your ult.