When Jesus painted these pictures of peculiar situations with surprising twists in the end, I think half of his purpose was simply to shift people's awareness. Surely one major reason he chose them to tell was precisely because they were each so oddly unique. But in Jesus' case, that unexpected twist wasn't merely good storytelling. It was part of the point about what Life with God is supposed to be like.
That, in turn, is why I don't understand how anyone can find "the" meaning of a particular parable. Take the Prodigal son. Does the Father represent God, or is he just a father whose raising the bar for the rest of us as parents? Is the prodigal son supposed to be Israel, or does he represent individual believers, like the lost sheep and the lost coin? Likewise, who is the good brother - Israel's leaders? Or anyone who looks down on God's struggling children?
Yes. Maybe. And not necessarily.
Now, you all know I am NOT the kind of guy to say 'well this passage just means whatever it means to each listener'. But with parables, we have to remember that Jesus told these deliberately-difficult-to-interpret parables to crowds. In those crowds there were parents and children, blameless Jews and backsliding synagogue skippers, rich and poor, lazy and diligent, humble and proud. The one thing they all had in common was that Jesus wanted to get them more tuned in to what GOD wanted out of them at any particular time.
So Jesus left Parables open ended.
Can we say the same about Gospel writers?
Luke may very well have had in mind one predominant interpretation when he chose to include the Prodigal Son at that point in his Gospel. If so, then we may very well be capable of determining from context clues in that passage and before and after it, what Luke intended as the main theme of that particular parable. In that sense, scholars absolutely have grounds to debate "the" interpretation of the Prodigal Son, according to Luke.
On the other hand...
The moment any Gospel writer included a parable, no matter how they presented it, and whether or not they greatly modified it, that parable itself brought along with it - right there into the midst of the writer's own sculpted narrative - its whole literary self, its whole history, its original content and its broader package of meanings.
So we have two things. We have Luke's intention. And we have Jesus' intention. In case this needs to be said, that is no contradiction. Again, if Luke used the passage for one purpose we should still think that Jesus himself may have used the parable more broadly.
Now, finally, to Hays v. Wright on the Historical Jesus.
Here's what all of this means. On the one hand, N.T. Wright is probably correct when he suggests (implicitly, as Richard Hays pointed out at Wheaton) that Jesus may have meant more by the Prodigal Son than Luke seems to mean by it. On the other hand, this also means Luke's text cannot support N.T. Wright's assertion that a particular interpretation of the Prodigal Son *IS* what Jesus meant by the Parable. Wright's interpretation about Israel's Exodus may even be accurate, but we can't tell so from reading Luke.
In some ways, this illustrates the rock and the hard place of doing Christian History based on the Gospels. Hays is absolutely right that we MUST respect the individual voices of the Gospel writers and their intentions as editors/authors. But Wright is correct that there was a Historical action behind these Gospel narratives. The question is, how much of that History can we get at?
I think we can get to some parts of the Lord's History better than others, but I don't think a Historian can ever make much of an argument from Jesus' Parables, as we have them. In my best estimation, the Parables that Jesus spoke to the crowds, in his own voice and rendering, were largely intended to be left open, interpretatively.