The most recent debate of the Synoptic Problem resulted in a dead-lock: The best-established solutions, the Two-Source-Hypothesis and the Farrer-Goodacre-Theory, are burdened with a number of apparent weaknesses. On the other hand, the arguments raised against these theories are cogent. An alternative possibility, that avoids the problems created by either of them, is the inclusion of the gospel used by Marcion. This gospel is not a redaction of Luke, but rather precedes Matthew and Luke and, therefore, belongs into the maze of the synoptic interrelations. The resulting model avoids the weaknesses of the previous theories and provides compelling and obvious solutions to the notoriously difficult problems.
– From here
This actually makes a lot of sense. If canonical Matthew and Luke are reimaged versions of Mark, why not include Marcion’s version as well? I think later Christians assumed that Marcion’s gospel derived from Luke because of Marcion’s reverence for Paul, and Paul supposedly using Luke’s gospel. So if Marcion used Paul’s “gospel”, then he was using “Luke”.
The bold arrows (1, 2, 3) indicate the main influence within the synoptic tradition, “main influence” here meaning that the post-texts adopt not only the general narrative outline from their pre-texts but also display, at least partially, verbatim agreements. The bold arrow (2) states what is obvious: Matthew is basically a re-edition of Mark, although enriched with further material. The new element in the picture is the influence (3) from Mcn to Luke. On the assumption of Mcn’s priority, there is no doubt that Luke followed Mcn very closely: as far as can be told, Luke did not interfere with Mcn’s wording substantially. Mcn is, in other words, a sort of Proto-Luke.
The dashed arrows (a, b) indicate an additional but minor influence of Mcn on Matthew and on Luke. In some respect, (a) and (b) most clearly show the advancement of this “Markan priority with Mcn” hypothesis: with respect to the far-reaching conformity between Mcn and Luke, the dashed arrows (a, b) indicate a bi-directional influence within the double tradition: there are elements running from Mcn to Matthew and others from Matthew to Luke’s re-edition of Mcn.