I don’t think rez can be balanced as a cooldown ability, but I have full confidence that it can be balanced as an ult.
Mercy 1.0 did it wrong in three areas:
1: It was really annoying to play against because it had no proactive counterplay
This was a fumble right out of the gate, here. The biggest complaints about Mercy 1.0 before a certain streamer started a witch Hunt was that when Mercy rezzed, there was nothing you could do to stop it once it started.
This is an easily fixable issue. Just add a LoS check/cast time (and for the love of Jeff, revert that old i-frames buff!)
2: It wasn’t useful in higher elos
This one’s because keeping allies from dying in the first place is way more valuable to pros than trying to reclaim momentum that you’ve already lost.
Easily fixable by adding a healing component to rez so that it’s more flexible and proactive.
3: It “incentivized” “letting teammates die”
This one’s actually more the dev’s fault for a lack of foresight when designing Mercy’s kit, rather than the players.
They made a primary healer with NO “backup heals” to fall back on when her base healing was getting overwhelmed - unlike literally every other primary healer, who has some kind of secondary healing source on their E - and then they gave her an ult with NO way to help living allies and was ONLY capable of helping them after they died.
What did they think was gonna happen when a Mercy 1.0 player got into a spot where her base heals wasn’t enough to save a teammate, and her only available “plan b” is ressurection? Mug Ana for one of her nades?
This is again, a simple fix. Give Mercy an E ability that heals, in addition to the solution to problem #2.
I’m pretty peeved that the devs didn’t even TRY any of these blatantly obvious solutions to Mercy 1.0’s problems.