[Edit: Renamed the title to better match what people call it.]
These would be specs where your hero goes through a questline to forsake their path and embrace a new source of power. They wouldn’t be 4th specs, but rather thematic changes to existing specs, giving the abilities new names and animations. It would be a good way for Blizz to create playable class identities that they otherwise wouldn’t turn into its own class.
Some examples would be:
Retribution Paladin embracing the void, essentially the plate equivalent of a shadow priest.
Elemental Shaman cursing the elements, using dark shamanism to submit them.
Demonology Warlock surrendering the fel and adopting necromancy, raising the dead rather than summoning demons.
Most of those examples would be better off as new classes. It would make zero sense for paladins to suddenly be wielding the void. It goes against everything the class is.
I think they should do it by racial identity.
For example, a Nightborne Warrior with arcane particle effects sprinkled into the animations. This can be applied in many different ways to many different races and classes.
That’s the point. You’d be a Paladin who either forsakes the Light or is left by the Light. In its absence, you’d turn to the void. Of course, you could just not do the questline if that thematic doesn’t interest you. It’s just animations anyway.
But then they’d be able to change spec back into one of the other three specs. Because Blizzard will never lock a player from a classes specs. A paladin is infused with the light. Adding the void would cause them to explode. That’s why void elves can’t be paladins.
You see right there what you said “a paladin that lost the light”. If that happens they are no longer paladins. Arthas lost the light and became a death knight for example.
Void Elf paladins will never happen if you want a high Elf Paladin, the horde is waiting for you.