Slitherine

It seems that the game will integrate Space Marines, Imperial Guard and Orks. But it there a possibility that further DLC and campaigns will add other armies such as Eldars, Tyranids, Tau, ... ?

From what I've heard it appears that they are going to add the Third War for Armageddon as an expansion and also an expansion that focuses on Titan based warfare. That's it though as far as I've heard for expansions.

I imagine that there will be some mods out eventually incorporating the other armies too.

I just hope that the IP they got from Games Workshop is not blocking any possibility of extension...