Well after the great ideas and feedback I got for my Tzeentch conversion plans I thought I'd pitch this idea as well. Haven't seen it used online yet (altough I would be surprised if this one isn't used by someone already somewhere)
I'm gonna use the Dark Eldar Slytth model as a base for my Herald of Slaanesh who will be Siren Songing in my army.
Here is a picture of how the model is before assembly:
As you can see the snake body comes without the head or arms attached to it. So all you gotta do is add 4 Normal Daemonettes or Seeker Daemonettes arms and a cool head from the Seekers kit and your done.
Pretty good right?