Most of the first 3 chapters of Revelation (John’s Apocalypse) are later interpolation. The blatant Roman Asia references (Ephesus, Smyrna, etc) reveal much about who the scribe was and where they insisted John lived; this makes the beloved Saint Polycarp of Smyrna a candidate for the interpolation, assuming Polycarp even existed. Alternatively, the author of the later elaborate prologue might even have been Saint Irenaeus of Lyon.
This means that everything from 4:1 onward are authentic, except for some of the more obvious interpolations, such as Revelation 12:17 and Revelation 14:12 [Rationale: I don’t believe there were explicit references to Jesus in the original Revelation].
The first words of chapter 4 give indication that there was some prologue prior to the authentic part:
After this I looked, and there before me was a door standing open in heaven
The above would certainly be an awkward introduction, considering the phrase “After this” is a transitional phrase, indicating something preceded John’s encounter with heaven’s door.
I think the entire original prologue can be found in the first chapter of Revelation. Below is my reconstruction of what I think the earliest Revelation prologue was (using NIV translation):
(Revelation 1:10)
[I was brought to the third heaven and ] I heard behind me a loud voice like a trumpet
(Revelation 1:12-17)
I turned around to see the voice that was speaking to me. And when I turned I saw seven golden lampstands, and among the lampstands was a man dressed in a robe reaching down to his feet and with a golden sash around his chest. The hair on his head was white like wool, as white as snow, and his eyes were like blazing fire. His feet were like bronze glowing in a furnace, and his voice was like the sound of rushing waters. In his right hand he held seven stars, and coming out of his mouth was a sharp, double-edged sword. His face was like the sun shining in all its brilliance.
When I saw him, I fell at his feet as though dead. Then he placed his right hand on me and said: “Do not be afraid. I hold the keys of death and Hades.”
(Revelation 4:1)
After this I looked, and there before me was a door standing open in heaven.
Tim Stepping Out 
I don’t know if you agree with me on this but I think the potential sources of Revelation or Proto-Revelation are probably pre-Christian sources given the themes of end-times tribulations, prophetic voices and addressing the Churches in Smyrna, Ephesus, Pergamum, Thyatira, Sardis, Philadelphia and Laodicea are in reality, pre-Christian Churches/Brotherhood groups or gnostic Churches since the word church or “εκκλεσια” just means assembly or townhall meeting (kinda like the RNCinCLE or DNCinPhilly).
Then the creepy part in Revelation 17:1-18 as the whore of Babylon. Many Christian denominations have come forth and creeped the entire world (e.g. Harold Camping, Jehovah’s Witnesses, etc). I think this entire talk about the desolation and whore of Babylon is nothing more but the Roman empire who led the Jerusalem siege in 70 AD. Then they combined it with the OT midrash of Isaiah 1:21; Jeremiah 2:20; Jeremiah 3:1–11; Ezekiel 16:1–43; and Ezekiel 23 as well as Matthew 24. It could be that the proto-Revelation is originally from Cerinthus but Church Fathers deemed it heretical so they took Josephus’s Jewish War, OT and the four gospels. I forgot where I read it but even then, some Church Fathers consider Revelation to be a heretical text.
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I absolutely do agree that proto-Revelation is either pre-Christian or captures the very earliest inklings of Christianity, although I don’t think Rev 2 and 3 are early – I think they were later Johannine community additions.
Yes – there were church fathers who thought Revelation was heretical – Caius the Presbyter was one prominent one.
There is so much of Revelation that is explicitly polemical against Rome, Nero, and Vespassian, that it does make sense that this text emerged out of a Jewish mystical community which was becoming increasingly anti-Rome and looking for alternatives to their Judaism.
My particular position is that Christianity provided the Judaism v2 solution via Jesus *and* the Docetic notion that a different God was above the Abrahamic one, and that he (not Yahweh) sent Jesus.
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Although it is probably about the Jerusalem siege in 70 AD, it kinda crossed my mind that it could may also be about the the Bar Kokhba revolt in 135 AD. Since the author of Revelation could have used Matthew 26, and Hermann Detering makes his case that the war described in Mark 13 is not about the Jerusalem siege but about the Bar Kokhba revolt (THE SYNOPTIC APOCALYPSE (MARK 13 PAR): A DOCUMENT FROM THE TIME OF BAR KOCHBA, 177ff). I’m purely speculating at this point but given that Christians have to pander with Jews who lost their temple in 70 AD, I’m still leaning that the whore of Babylon is all about the atrocities committed by the Roman Empire.
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I would not rule out that there were several authors over several decades. A Youtuber (Xoroaster Zoroaster) made a series of videos (Decrypting Revelation) arguing as much. https://www.youtube.com/watch?v=RJbRQrvG0kE
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