Has there ever been an attempt to collect those Āgama NOT held in common amongst the classical cultures as a way of looking at the diversity? Even as a topic of an essay? Something that would illustrate the real depth of field in the Canon? The 2 "popular" books I found on the "Sutras" on Amazon are both wonderful - but on the details are frustratingly less fulfilling than a Wiki binge, and the footnotes referencing distinctions in the academic texts I've found are, well, scattered leaves. Is there a Graduate text of "Buddhist Sutras" that is a survey course, not linguistics?
It's sections could be classified as the source where this otherwise unattested material was located.
1.) It would be a sampling of all the sources as being a spectrum of representation (cultural anthropologists and language specialists would like this),
2.) it would illuminate what was distinctive to each source, (historians of religion would like this),
3.) and it might be amenable to an exercise in translation which would also forge a common language for a divers group of specialists, (ways words meant different things in different groups settings would shine a light on the common root sense).
Mizuno, Kogen. Buddhist Sutras: Origin, Development, Transmission. 1st English ed. Kosei Publishing Company, 1989.
Williams, Paul. Mahayana Buddhism: The Doctrinal Foundations. 2nd ed. Routledge, 2008.