Actually, I mentioned this in another thread, but now that I think about it I definitely sense a lot of GDW's old titles becoming source material for theme and tone of the current WH40K RPG titles. As I said elsewhere, Rogue Traveler looks like it shares concepts with Traveler (wartorn universe, commercial trade, etc.). Dark Heresy is very similar (incliding in title) to Dark Conspiracy (like many horror games, except guns work on the majority of the bad guys, but in the end it takes investigation, teamwork, and large-scale operation to defeat the real evils - also, the evils can be defeated, even if they can't actually destroyed).
How much do you want to bet we'll see a bit of Twlight: 2000 (or Merc: 2000) in Deathwatch? Which will be great - T2K is something of a model for how to set a roleplaying game in a military environment during wartime.
Hatred has purity.  Hatred has size.
Hatred has purpose and hatred has eyes.
Hatred has honor and hates to hear lies.
Hatred has energy.  Hatred is near.
Hatred has carnge and hatred is clear.
Hatred is frustrating love for fear.
Hatred is human and hatred is here.
-Ship of Fools, The Residents
Seems an age ago, but can I just thank Kage for setting me right on my rant about the setting - I think I conveyed my feelings on this a little unclearly!
I just wanted to say, without derailing the topic too much I hope - that I was excited when the DH system came out, simply because as both a role player and a tabletop player as well, this was a highly awaited project - it was unfair of me to suggest that this system is the only definitive way that you could enjoy the setting.
Now - back on topic! I feel that many older role-play systems could inform the development of this - as I have probably said before, CoC was for many years my favorite system, and coupled with T2K or Dark Conspiracy would help to explain the war torn, hopeless fight humanity has on its hands.
We can always only speculate about the eventual power levels within the game, but a gradual progression upwards through Deathwatch, and also a similar difficulty setting in terms of opponents and narrative threads would seem to be keeping all parties happy - and provide enough different aspects of the 40K universe to be experienced to keep most people happy. True, you may have to rig up your own rules to play a Tau pathfinder, but there are enough mechanics to play with to work out your own offshoots if you feel the need.