It would be cool having bigger chances of reviving someone without the mobs finishing players off, but since making them invulnerable like in hardcore revival leads to cheesing the mobs, I think making them ignore downed players would be a good alternative, not making them totally invulnerable by still taking indirect damage sources like lava or burning surfaces, fall damage etc.

This is something I want to do eventually, but it isn't so simple unfortunately for a variety of reasons.

I would need to change the AI of every mob in the game, both vanilla, and from every given mod essentially, to have them care about the downed state of the player. Were it just vanilla mobs I would need to be concerned about this would be a tedious but potentially manageable task, but with modded mobs in the equation, and the many mechanics that may be involved that I may incidentally overwrite, it is not a particularly easy feat, unfortunately.

That said, I may have an experimental way this could maybe work, but it seems very... potentially destructive itself.

maybe some way to switch the player to spectator or creative while keeping them on their place like you can do when right clicking mobs in spectator mode, leaving a model behind for others to revive, then bring back the player to survival when helped?
sadly this would make them invulnerable but at least mobs would ignore them

Yea, this is similar to what I was thinking.

Iirc in the code, there is a method that checks if the user is in creative or spectator. Instead of throwing a user into one of these game modes and potentially allowing them to do things they shouldn't be able to, i could potentially mixin into that method and add a configurable option that will also have that check if the user is down.

I'm still thinking on if this would cause any major issues, I don't entirely know if it would in either direction. But I can't say for certain without knowing how every mod ever interacts with that check.

Suppose with making it configurable I can at least warn users that it is somewhat dangerous of an option and may lead to some unexpected consequences