The Horus Heresy I think is pretty unique in that nearly every 40k player knows about it, even those that have very little interest in fluff. It has been part of 40k for a very very long time.
I can't say I have much contact with the fluff, but all of the battles(?) listed so far I haven't even heard of, I would say I have an average interest in fluff and a low interest in marines - not sure how typical that is, but if it is then there would be less interest in models for such events.
I think it much more likely we see some of the major characters (as mentioned) the very scenic show-models which have been made for the primarchs are a good example of what FW can make and sell to the fluff fans and they break into the less tapped "painter-modeller" crowd who don't care about the games just paint nice models from any manufacturer.
As for the GW support role that FW fullfills, I see more upgrades/bits sales - HH is a good model for what can be done. Niches like as Djinn said the missing phoenix lords for Eldar and of course the ever present big tanks element.
The final avenue which I expect to see in the next few years is another non-imperial FW only army, Chaos dwarves shows how it can be done and if there is an option for FW then they will do it, one niche appeal army which would lend itself to highly detailed models and tanks would be mechanicum (which they seem to be tentatively exploring) or maybe even the highly ignored sisters of battle?