A parable is a story that teaches a spiritual truth. In order to write a modern parable that shows the message of the Parable of the Prodigal Son, or the Parable of the Good Samaritan, you first need to understand the meaning of those parables.
In the Parable of the Prodigal Son, a young man demands his inheritance of his father and leaves his father's home. This was a grave insult to his father! After some time of wasteful spending and riotous living, the son finds himself alone. His money is all gone, and he has no friends. He eventually sells himself into slavery to make ends meet. Ashamed, he comes to his senses and returns to his father. Hoping to be made a servant, he is amazed when his father forgives him and welcomes him back with open arms. The parable is intended to convey God's love and forgiveness for those who return to him, no matter what they've done.
In the Parable of the Good Samaritan, a Jewish man is beaten, robbed, and left for dead on the highway. Several people pass by him, including a priest, yet do not take the time to help him. Finally, a Samaritan walks by. This is significant because there was a great deal of prejudice and animosity between Jewish people and Samaritans. The Samaritan stops and cares for the man, even paying for his care while he heals. The parable shows that we should have compassion and love for everyone we meet, regardless of who they are.
To show these parables in a modern setting, you would want to update the setting while still keeping the message intact. For example, the prodigal son rejected and insulted his father, then hit rock bottom before returning and being forgiven. What would that look like in your culture today? Imagine someone in your culture who was beaten and left for dead, like the man the Good Samaritan helped. Who would be the least likely person to show him love and compassion?