That being unlikely, a Horus Heresy one that used Only War style rules and had ways for both Space Marines and others? Yeah, that would be perfect!
Given how both the HH novels as well as previous attempts to mix Astartes characters with other people in FFG's 40k RPGs worked out, do you really think that such a game would truly be fun to play for everyone? I mean, people who would opt to play a human would basically be pushed into the role of "supporting cast".
Most fans craving for a HH RPG (and I do agree that it'd likely be a lot) would probably like it to stick to the vision propagated by the popular novel series, meaning a Deathwatch-style epic experience focused on how great the Space Marines are, complete with that ton of special rules that throw bonuses at Marine PCs like there's no tomorrow. I'm not sure normal people have any place in such a game other than to serve as extras. Black Crusade was already pushing it, but there some classes at least had the option to stand out thanks to their psychic powers, special influence, or the ability to have mighty daemons and CSM's as NPC minions. A Heresy RPG, on the other hand, would take place at a time where Marines run the entire Great Crusade, and I don't think summoning daemons would be an option to counteract that immense gap that has been introduced with the previous games either. Sure, in theory they could close that gap again, but that would surely go against expectations of a great many fans as well as the narrative produced in the HH novels.
And given the previous games, I have also come to be rather sceptical regarding any RPG from this studio that brings Astartes and normal humans together. For a long time I used to hope otherwise (I'm a big fan of the "one rulebook for all" idea), but I've come to believe that under the circumstances it may truly be better to keep them segregated into different games. It simply comes down to people having to decide whether they want Marines to be that awesome that they put everyone and everything else into their shadow, or whether they'd like a game where they can actually fight side by side with non-Marine combat characters (as may happen in some novels or Codex fluff) and have the latter still enjoy things. It's one or the other, but not both.
Granted, designwise many of the perks that make it difficult to reconcile both types of characters come down to the underlying system (I'm looking at you, Unnatural Stats and combat rules), but the studio already had the chance to address these in the past and produce something more intercompatible like GW's Inquisitor game, but aside for slightly toning it down for BC they really didn't. I don't see why that would change now. It quite simply seems to be a matter of one's interpretation and personal preferences, but the abovementioned limitations apply.