Druids in classic are a very specific class, because healer is the only viable role and even as healers they’re worse than other classes. Also, since druids are HoT-heavy healers, and HoTs don’t stack in classic, they get even worse when there is more than one druid in the group.
So I’d suggest replacing the rebirth spell with something that would make druids more meaningful. Most people would suggest something like Death Evasion from Diablo 4, but I’d take a different approach - give druids the ability to resurrect after death with long cooldown (like 100 days) and 7 days of resurrection sickness to ensure one doesn’t hop to another raid right after a wipe.
The reason behind my way of thinking is that preventing death of one group member wouldn’t work on hardcore, because you never know who to apply the spell on - the tank, who will almost certainly not die unless the healers die, in which case the tank is doomed anyway - or one of the healers, and take a guess which one is going to need it? Also, it’d discriminate damage dealers, because nobody would ever DE a DPS, and on hardcore realm it’d be very important to be the one who gets a second chance.
So giving druids the second-chance ability would be something unique to the class, which is in the spirit of Classic, where advantages and disadvantages of a class aren’t limited to the ability to tank, heal, or deal damage. It would make the druid class more attractive to players, giving them the advantage of second chance at the cost of the disadvantage of lack of demand in raids.