I can read this parable as an illustration of repentance and forgiveness in a family. The younger son has sinned and recognized his sin. Or, at least, he has recognized that he needs his family. Admitted to himself not yet to those he has sinned against. But, before he can do that, his father comes to him, comes not reluctantly or grudgingly but running.
His father embraces him. Then, the son speaks his words of repentance.
Or, paying attention to the introductory words of this chapter about the grumbling Pharisees and scribes, I can read this parable metaphorically. Who is welcome at the table? What prevents someone from being worthy of sharing a meal with us? Who gets to decide? Which comes first--repentance or grace?
Further, what is this grace for, anyway--and, who's it for? Remember that Abraham was blessed to be a blessing (Genesis 12:1-3). Remember Jonah's assignment. (Thanks to Allen & Williamson, once again). Also, remember that foreigners had been included in the Exodus (Numbers 9:14; Judges 1:16)