This seems plausible enough; and I don't know why people scoff at it. This wasn't written by me, but by a “gurugeorge” on FRDB:
I don't doubt there are some parts of the synoptics that might have been part of the older visions. The movement was already 40-50 years old by the time GMark might have been written. My idea of the synoptics is as follows. (Bear in mind that I'm holding the following true for the purposes of this interpretation – the standard view which has Paul as the earliest, and the results of Bauer's investigations into the actual composition of early Christianity):-
approx 35 CE – 70 CE – no gospels at first. Tiny but widespread movement based on middle-class “New Agey” mysticism and occultism around a postulated Messiah found prophesied in Scripture, who had already been and done his work, and was now contactable in spirit vision, and mystical-union-achievable-with as a deity. What's produced in sessions is (amongst other things) messages from the god about it's doings while sojourning on earth. Some story elements become popular, there are a few sketchy “gospels” or biographies floating around. Towards the end of this period, one of them, an “ur-Luke”, becomes popular, perhaps mainly as shared, loose oral tradition at that stage. EVIDENCE: If Paul and Hebrews earliest: the absence of evidence for the existence of a human Jesus in Paul; the presence of evidence for belief in a historical but divine being; the presence in Paul of a clear description of mystical and occult practices as “what we do” in Corinthians 12-13; reading “according to Scripture” as meaning reporting; it's clear from Paul himself that he is talking about a once-historical, now-visionary entity that talks back to him, which is linked with a mystical experience of connection with that deity (or rather, strictly speaking, revelation of the always-present connection); scholarly investigations which show some parts of Luke are not accounted for in the standard hypothesis, and appear to be very early; later on, Marcion reportedly uses some kind of slimmer “Luke”, later Gnostics are known to have traditionally favoured “Luke”.
70-90 CE – Someone or some people write GMark – which takes the most popular story-skeleton (the hypothesised “ur-Luke”) and rewrites it as a dramatic gloomy post-Diaspora retro-prophecy. The idea is introduced (or is a re-emphasis of an earlier speculation, perhaps in the ur-Luke) that some of the original apostles knew and walked with the cult figure. At that time, we'd naturally expect a variation of takes on the theme – some holding the cult figure more like in John, a true superhero-type, some taking the cult figure to have been more like in Mark, more like a preacher or priest while on earth, but certainly a vessel of the Divine in some sense). GMark is based on a more humanized vision of the saviour. Quite innocent, just natural drift in interpretation. One school or sub-sect, probably not the one “Mark” belonged to (which is more traditional proto-gnostic) but a sect sharing a more humanised vision of Jesus, but also a more Jewish-favourable stance, picks up this idea, and drafts a GMatthew that somewhat orthodoxises and “catholicises” Mark. This becomes the central gospel of the new orthodox movement. EVIDENCE: the orthodoxy self-ascribed GMatthew as being their earliest and most popular gospel, yet we know from scholarship that it can't have been; the scholarly work on GMark shows little that could be construed as apparent quotes from a human Jesus, but a whole ton of stuff based on Scripture; the absence of evidence in Paul (presuming him earliest) that any of the apostles before him knew the cult figure personally, again combined with the ever-present absence of external or internal evidence that would support a man Jesus.
90-150 CE – GLuke and Acts fabricated in response to the threatening popularity of Marcionism (still a small movement though, only a few thousand folks at this stage, still a relatively well-to-do and middle class affair on the whole). GLuke based on the “ur-Luke” used by Marcion, but taking material from GMark and GMatthew. Acts uses some folk-memory stories about Paul. Kerygmata Petrou an alternative kernel for Acts that's binned as being a bit too Jewish-weighted, and a bit too fanciful. By this time, orthodoxy is beginning to appear historically (cf. Bauer) – and it's fighting the already-established variegated, more or less woo-woo descendants of the original Jerusalem and (mostly) Pauline forms, wherever it goes. Towards the end of this period, GJohn is written, perhaps based on an earlier text more obviously Gnostic (cf. Doherty on this). Meanwhile, lots of other gospels and other material start being written by many different schools, partly in response to the first two, partly as a natural effusion. EVIDENCE: there is scholarly belief that the two are the work of the same hand; the tenor of these two documents is Catholicising and has always been recognised as such; Acts is one big exercise in reinforcing the idea of the Apostolic Succession; yet Paul has to be accounted for somehow, those who still follow him have to be “kept sweet”, so “Peter” and “Paul” shake hands; Bauer shows a to-and-fro struggle between orthodoxy and heresy – it may be the case that GJohn is a further attempt by orthodoxy to get Gnostics on board, i.e. take a gospel that's popular with them, and Catholicise it.
150 – 200 CE – by this stage, orthodoxy is starting to really flex its muscles, it has the power and money to gradually unify the Christian movement around its version of the myth, which it increasingly pushes as “canon”. It's also lucky enough to have some sharp, rationalistic minds on its side. No more gospels need to be written, gospel-getting, prophecy, occult practices – the very stuff of the Christian cells as originally seeded by Paul – are curtailed and eventually outlawed.
200 – 400 CE – the movement is gradually positioned as a mass-movement, grows a bit more, and by the end of this period is eventually presented, neatly trimmed and prettified, to Constantine as a viable possible religion to unify a failing Empire.