Hopefully people want a fluffy battle.
Fluff wise, There are space marine chapters that go renegade, but do not go over to chaos, they set up their own territories and rule until the imperium finally notices and crushes it. Same with Imperial guard, etc.
In fact, Chaos fight each other all the time, it's not unthinkable that a tzeentian warband would attack a force of khorne beserkers while they were engaging the imperium.
Really, the fluff is more or less written in a way that guarantees a good reason for any two armies to have it out with each other.
I do agree though, that this is something worth discussing -- On one hand, I'd love for everyone to be able to field the army they want and to play with the friends they want, but I can definitely see how the forces of Order get the shaft in this regard.