That's a good question. I don't know the answer, but...
In the Empires series they always added large pre-set historical battles (Like the Yellow Turban Rebellion) that you could join. In a 'Warriors Orochi: Empires' there would be no historical battles so they will have to create a lot of battles that never really happened.
The amount of Characters is crazy too, the Japanese rulers would be Nobunaga, Tokugawa, Hideyoshi, Kenshin, Takeda, Masamune, Mitsunari, Chosokabe, Azai, Hojo, and Imagawa.
Add to that the Chinese leaders: Cao Cao, Liu Bei, Sun Jian, Dong Zhuo, Zhang Jiao, Meng Huo, Sima Yi, Yuan Shao, perhaps some more I missed.
That's a lot of leaders and they all need their own territories. Perhaps a map of combined China + Japan? Or an entirely new map? Either way it's quite a bit of work. And with the release of Dynasty Warriors 7 both the characters and the graphics have received a bit of an upgrade. I think it just wouldn't fit on anything short of a blue ray DVD.
But I agree with you that Empires is amazing. It's my favorite of the Warriors series. And a Warriors Orochi 3: Empires? Yes, please.