I’m not planning on playing Soil Harvester because the fantasy of Heckcaller is a lot more alluring to me.
But it’s kinda weird that you chose the model of a Void Revenant for the Demonic Entity that possesses Soil Harvester Warlocks, even though you’ve said time and again that Void beings aren’t true Demons.
I’m not saying “remove this one”, I’m just saying maybe a Fel alternative would be a positive thing 
My soil harvester is down on their farm in pandaria.
Maybe give warlocks the option to have an enslaved Demon Hunter as a fel-alternative? 
“This is your punishment, for stealing our metamorphosis!”
/s
Nope!
I can’t imagine a worse fate than having a Demon Hunter living inside my soul, whispering “I’ve sacrificed everything, what have you given?” into my brain all day long…
You’d be enslaving them, not the other way around.
Also — If you got tome of sacrifice demon, after you use it - You could step over their dying body, look down on them and say:
“Now … You have sacrificed everything.”
Jokes aside — It’d be cool to have a fel revenant, like there was in the Hellfire Citadel raid. 