TBH with the heresy they really could do anything they wanted within the timeline starting with the unification wars and go all the way up to the great scouring. I mean unification has already been in the series with the (excellent) Last Church short story in Tales of Heresy. And you really can't expect the series to end with 'and the emperor pimp slapped horus into oblivion, the end'.
Also, they can invent a lot of events out of nowhere. I mean What happened in some of the books, Nemesis, for example has never been covered with the fluff. I mean there were 18 legions, there could easily be a book on each legion before, during and after the heresy during the great scouring. And the imperial Army has only really been a major factor in Legion, they deserve more of a role. Remember, a third of the Imperial Army fell with horus.
AND there is the Age of darkness, the 7 years where nobody knows much about. The thousands of other battles for worlds during the heresy. The time period of 20k-32k could all be covered during the heresy. Hell we might even get an Emperor origins book, and when was he born? Like 3,000 bc or something like that. They could wrap the series up in 5 or 6 books if they just wrapped up the major events, but they could easily do another 15 or 20 or 50.... buy I don't know if they would dilute the series 'special collection' appeal.