I wouldn't mind borrowing a trick from Mech warrior. You might want to throw out a challenge like this.
"I have a 1500 point Grey knight army with 1 HQ 3 elites 3 troups and a fast attack - prepared to take on any Xenos army but the lowest point total army gets the first shot"
Give people a week or so to design a list and submit bids.
This could represent an attack on a homeworld or outpost, and give the math hammer players some interesting analysis to do, and you some story and fluff.