So, what enemies do you think will be the main ones in the new Game? In my experience Dark Heresy had Chaos and Hereteks (and alot more but I thought those two got the most focus), Rogue Trader had all manner of stuff, Deathwatch had Tyranids, Chaos and Tau while Black Crusade seems to have the Eldar, Necrons and Imperials as the main enemies, and other Chaos followers of course. My basic belief is that they will do it kind of like Deathwatch in its set up.
So what I think, and hope, is that the Orks will have a place as the Imperial Guard vs Orks are seems just as iconic to me as Imperial Space Marines vs Chaos Space Marines are.
Secondly I would think that Chaos will have a part as they seem to have played a pretty big part as enemies in all the ealier Imperial lines so Chaos in one form or another seems like a good choice.
For the third I would imagine that there will be the Dark Eldar or Eldar honestly. My reasoning for this is that the masses of enemies coming screaming at you-thing is done by the Orks and the utter evil is done by Chaos. And that means that I think they'll have some enemy that can possibly be bargined with and also will fight very differently from the other two and so offer different scenarios. Possibly the new Necrons could do this as well but they are not awakening yet according to the official timeline and since I think that the new Sector, and I think it will be a new Sector, is likely to be close to the other parts of the line I think that the Tau are to far away. If they made some entirely new xenos race to face the Imperial Guard that would be very cool but I think they will go with the great, well known and iconic races for the main enemies.
That's some loose thoughts on the issue. I'd love to hear some of what you other guys think.