A thought came into my mind about the son of promise. Abraham is the father of faith and he had to be willing to sacrifice his son, just like GOD would do later with His Son to save the world. We know that GOD was just testing Abraham's faith in a way only GOD can fully understand.
The thought about the promised son of GOD to Abraham that came into my mind today, is that Isaac is the one that lived out the true Biblical defined marriage with Rebecca.
Abraham slept with Hagar and had a child outside of marriage. Jacob (Who would later be named Israel) had two wives and two concubines.
Issac had Rebecca and that was it. Two became one with Issac and Rebecca as GOD teaches us about marriage in Genesis 2:24. I don't exactly know if this is significant, but it came to my mind today and it seems at least note worthy.