I was just thinking, "What would have happened if Onesimus had written a letter to Paul after he had been back at Philemon's estate?". Perhaps it would have gone something like this...

(Six months after the letter of Philemon is written.)

"To my beloved brother Paul from your pupil, Onesimus:
Greetings in Christ’s name. I write you from Philemon’s estate, to which I returned safely several months ago. I trust that you are in good health and have continued to bless your servants with your letters as you have somehow mysteriously blessed mine. If you would greet all of the brethren for me, I would be most grateful.
As per your request, I did not read the missive that you sent to Philemon. I would not dare to break your confidence in such a way. However, over the past few months I have noticed a marked difference in the way that Philemon has treated me. When I arrived, I was pensive. Even through all of our long discussions concerning what I should do, I was still nervous about what would happen when I faced my master. Upon my arrival, the letter was stripped from me and I was roughly escorted into his presence and quickly subjected to a lengthy tirade from Philemon. I have never seen the man so angry! Once he finished he walked to a table upon which your letter had been placed. The entire experience was extraordinarily uncomfortable for me, due to the guard’s hand resting upon my shoulder for the full duration. This is when the change began to occur. Upon seeing that the letter was from you, he instantly became sedate. I have never seen a man change as quickly as he did. It was…surreal. He quickly broke the seal and began to read. With each passing moment, it was as if Philemon became increasingly contemplative and cheerful. Upon finishing, he slowly took a seat in a nearby chair and looked at me for a short while. He then told the guard to escort me to my room. Yet, the guard was told not to guard me. I could not understand what had happened! You know well enough that the penalty for running away from a master is severe punishment, to the level of death in some cases. I was simply dismissed and sent to an unguarded room with instructions to meet Philemon the next day during the morning meal.
I will admit; at this point, I was confused. I am by no means a man of learning. In fact, I have one of my master’s scribes penning this for me. (I have been learning! But I still need time yet to become able to write my own letters.) Well, the next morning I met Philemon. Oddly enough, his wife was there as well. (He usually eats his morning meal alone.) After I sat, there was a short, awkward moment, and then Philemon began to speak. He and his wife both explained to me that, although I had betrayed their trust by running from them, they were going to give me another chance. He told me that he was going to begin to teach me the intricacies of a noble’s life and to place me in charge of parts of the estate. He then told me that I would begin my new duties the following day. As a final word, he told me that he and I would be having a long discussion each evening. About what, he did not specify. Then they let me retire. I was dumbfounded, and quickly bowed my way out to attempt to understand what had just happened.
It has since been….six months, I believe. Philemon and I have become close friends and our relationship improves with each passing day. I must admit that I am still completely taken aback as to what occurred to Philemon. All that I know is that your letter to him was the cause of this. I am now not only in your debt spiritually, but socially as well. God has worked something through your letter to Philemon. I thank you with all of my being. If you are ever in a position to need something that someone in such a lowly position as me can provide, please allow me to aid you.
Philemon and his wife send their greetings, as does your flock. My mentor, I hope to meet you again should God will.
With regards,
Onesimus."

Philemon 1:1-25