If they brought an a380 at all I have to think it would be a 2-class. I have a hard time believing IAD could support an additional 8 F seats over the current 777 - though the a380 2-class plane is likely wayyyyyy too much plane for IAD.

Seems more likely to be an early candidate for either 787-10 or a350-9 depending on which EK ultimately orders (assuming in either case J will feature lie-flats) which would match EY.