In very general terms, there are 2 broad revisionist theses regarding the Quran:
1. Some of its content predates Muhammad, generally viewed as being part of Syriac Christian lectionary.
2. Some of its content post-dates Muhammad and reflects later editing of the text over the next 100-200 years
Examples:
The Qur'an and its Hypertextuality in Light of Redaction Criticism (The Qur'an and its Hypertextuality in Light of Redaction Criticism)
Guillaume Dye
This paper argues that the application of the tools and methods of Redaction criticism to the Qur'an can yield extremely fruitful results.
Studying, more specifically, surah 19 (Maryam), it shows that this surah displays several layers of composition. The earliest layers are described as a "text of convergence" with Christians. It is shown that all the details of the text have their source in written, liturgical and popular Christian traditions, most specifically those related to the Jerusalem Marian liturgy of the early 7th century. Analysing the profile of the author shows that this surah was most probably composed after the conquests by a Christian monk who "converted" to the faith of the newcomers, or put his pen at their service. It also show that Q 3:33-63 is later than the early layers of Q 19:1-63, and that Q 19:34-40 was added to Q 19 later than the composition of Q 3:33-63.
This raises interesting questions about the chronology and composition of the Qur'an. Solutions to various specific riddles in surah 19 are also suggested.
J. Wansbrough and the Problem of Islamic Origins in Recent Scholarship: A Farewell to the Traditional Account
J. Wansbrough and the Problem of Islamic Origins in Recent Scholarship: A Farewell to the Traditional Account [2012] / Book Chapter
I'm not necessarily making the case that either of these views are correct, I'm generally agnostic on the issue as I'm not knowledgeable enough to evaluate the claims sufficiently.
I've read a lot of stuff on this and at first you tend to go "Wow, that really explains it!". Then you read something else equally persuasive that points out, often highly technical, reasons why it is wrong. It's very much the more you know the more you realise that you don't know.
One person might say Quran is an etymon of the Syriac
qǝryānā, “reading of Scripture in Divine Service”, whereas someone else might make the case that this is unlikely to be true, and I don't have the linguistic knowhow to judge between the arguments.
So while it's interesting, I'm not going to pretend I know the definitive answer.