Well part of the original marketing scheme behind Horus Heresy (the first edition) was that it was one of the great historical battles of the 40K universe, along with Battle for Armageddon and Doom of the Eldar. One of the best things about the 40K universe in my opinion is the rich, textured fluff it provides alongside all its products. My fear is that there isn't very much you could do in the way of an expansion without elaborating or changing the "historical" details of this famous battle. Elaborating on it is dangerous to continuity and changing the details would be heresy (no pun intended. =P)
In the case of Armageddon, GW has been able to revise the idea by having several battles waged on that planet throughout the ages, but I don't see how you could make a second Horus Heresy. Instead of an expansion to HH, I think I'd rather see new implementations of DotE and (maybe) Armageddon, if GW isn't holding onto that one for their own uses.
I'm sure they could add some new units and some optional rules and what not, but I don't know that there's enough material to justify a whole expansion without shattering the established continuity of the 40K universe.
Edit: AND A NEW EDITION OF TYRANID ATTACK! ZOMGWTFLOLWIN. =P =D =P