Lately I’ve been thinking a lot about the concept of a prequel, and it’s clear to me that it’s a genre with deep roots. Oedipus at Colonus is a self-conscious prequel to Antigone, for instance, supplying interesting background to Antigone’s torn loyalties. (By contrast, Oedipus Rex does not seem to me to be so self-consciously prequelic.) A similar argument could be made for the Book of Genesis — though it draws on materials that may predate the Genesis story, it was redacted in such a way as to provide a self-conscious prequel to the Exodus events and the subsequent conquest of Canaan (including Israel’s relationships with its closest neighbors, Moab and Edom).
The most interesting prequel element is the Joseph story. It is very distinctive in its style and approach, as it injects the kind of post-exilic narrative we find in Daniel and Esther into Israel’s primal history. The most straightforward narrative work of the story is to account for why the Israelites were in Egypt in the first place. But it goes beyond that in making Joseph (and the Israelite God who so favored him) into the source of Egypt’s survival and Pharaoh’s overwhelming power.
In this sense, like the most ambitious prequels, it winds up recontextualizing and even subverting the narrative to which it is a prequel. Joseph engineers a plan to reduce the entire Egyptian population to slavery, and for his service, he gets the assurance that his family will be a privileged population. Once a subsequent Pharaoh forgets about this promise, he initially reduces the Israelite population to the same level as everyone else.
Obviously he goes much further than this later on, but the background of the Joseph story shows that the problem isn’t mass enslavement as such — it’s mass enslavement of the Israelites. A narrative of God’s justice and liberation retrospectively becomes a narrative of God’s favoritism.
An und für sich 
The first two chapters of Luke are also rather prequellic. Or maybe they consist of a sequence of prequels (“everyone liked the annunciation to Mary, let’s have an annunciation to Elizabeth too…”).
Interesting — it would count as a prequel because Luke assumes people “already know” the basic story?
The idea of a prequel roots itself into the epic (that is the genre, epic) *in medias res.* Suspense is palpable in classical epics, but it is of a different order than what we today identify as suspense. Epic suspense depends on what is already known, and hangs upon the presentation, the ‘when-ness’ of that presentation, in the narrative flow. So, when Exodus begins with a pharaoh arising in Egypt who knew not Joseph, tension and expectation are already in play: a national epic is afoot.
In addition to the general familiarity with the Jesus narratives which you suggested, Adam, there is the conventional assumption that Mark’s gospel was the first to be written, so it’s plausible to think of readers of Luke’s gospel as being already familiar with Mark’s.
I guess that would make Luke as a whole the deluxe edition with previously unreleased footage (including an origin story). But I think the first two chapters do function as a prequel, partly because they are full of people reacting to stuff they don’t know about yet.
Luke is definitely the Gospel Director’s Cut.
I wish I’d had that metaphor when I was at bible college.