By this I do not mean that there's not a great wealth of historical research being done or that prior histories (Barnouw, Barsam, Ellis, etc) are not valuable. To the contrary. In my eye, the historical turn, which I might associate with Wisconsin school historiography does a few worthwhile things that documentary studies would be well-served to take up.
Revisionism. We don't want revisionism for revisionism's sake but we should where possible interrogate standard histories to see if they hold up to the empirical evidence. This does not mean positivism: any such interrogation involves a creative reimagining that can be informed by theory.
Non-canonical approaches. We don't need to resist any attention to canonical films and film makers, but we should not assume them as the only or primary object of study. We should think about sampling and representativeness. We can draw connections between the extraordinary and the typical, or between documentary and its others (newsreel, industrial film). This is especially vital as new films become more widely available because of video and new media accessibility.
Application of methodologies from histories of fiction film. Documentaries may require their own methods at times but I'd like to see more reception study, industrial-institutional analysis, and historical poetics analysis
Inductive approach to archival research. This is already being done, but it is worth emphasizing.
Prestige of historiography. This is harder to argue since it's not a theoretical point, but one thing the historical turn did was elevate film history from the margins to either center of the discipline. I don't argue for this self-servingly but because film history requires a good bit of (often invisible) academic labor and because dismissals based on anti-empiricism were never justified in my eye. Visible Evidence grew out of the injection of film theory into doc studies and I would like to see a complementary inclusion of film history.
A lot of the above has been done and is being done. But to cohere into a historical turn I would argue for a self-conscious connection between these activities, one that's theoretically and methodologically confident. It's why I appreciated the Visible Evidence workshop on nontheatrical film, during which Joshua Malitsky took up the mantle of the historical turn to argue that we need to bring disparate parts of our field together, since they do have a lot to say to one another.