How about a battlebox, mixing light and shadow elves? They will be of two different armies, even before their battletomes - for people, wishing to get those models as soon as possible, so a battlebox will bea preview of sorts. On the other hand, logically, another army in a battlebox should be of Slaanesh followers. Maybe cultists and mortal troops with a champion like an old Sigvald. But then again, battletome of Hedonists already on it's way, so instead of Slaanesh forces those might be of Archaon - possible with the same "choose your allegiance" system, representing Slaanech followers in this particular situation. Does this sound stupid?