First off, let me start off by saying that I am looking forward to this game. I am a bit of a game collector/junkie, and I think this has potential.
Now, that being said, my critcisms of the idea-
1. Flesh out the other game lines first. Granted, they can take a hiatus from DH for a while, as they have most of it covered, and even have not one, but two trillogy adventures published. I wouldn't mind seeing some more material strictly for Ascension though. RT has quite a bit out for it at the moment, so they could get away with a breatk for bit a well. Deathwatch still needs a bit more, namely a nice big epic adventure. BC obviously needs much more done.
2. Xenos. Yes, the Imperium is awesome, I myself am a hardcore Space Marine player. That being said, there are hard-core fans of the Xenos races who should get their turn. The options in "Into the Storm" are nice, but you are still essentially playing an Imperium based game. An Eldar game has potentiel, as does even an Ork game if done right (I could see adapting the BC model where your eventual goal is to gain enough support that you lead a WAAAGH). A Tau game could even include more than one race, as the Tau Empire is a tad more cosmopolitan than the other stellar nations. I think a game focusing on Xenos should be next instead of Guard, personally. Granted, players of Nids and Necrons are going to be left out, as there really isn't much that can be done RP-wise with them, though a game focusing on a Genestealer cult DOES have potential.