You have to read it within the whole context :
15Then God said to Abraham, "As for Sarai your wife, you shall not call her name Sarai, but Sarah shall be her name. 16"I will bless her, and indeed I will give you a son by her. Then I will bless her, and she shall be a mother of nations; kings of peoples will come from her."17Then Abraham fell on his face and laughed, and said in his heart, "Will a child be born to a man one hundred years old? And will Sarah, who is ninety years old, bear a child?"18And Abraham said to God, "Oh that Ishmael might live before You!"19But God said, "No, but Sarah your wife will bear you a son, and you shall call his name Isaac; and I will establish My covenant with him for an everlasting covenant for his descendants after him 20"As for Ishmael, I have heard you; behold, I will bless him, and will make him fruitful and will multiply him exceedingly. He shall become the father of twelve princes, and I will make him a great nation.…
The context of the passage clearly shows god telling Abraham I have heard you meaning heard your prayer and accepted , So god blessed him and made him a great nation and as I described a great nation before from the bible is one which has a law and worships god and blesses Abraham. This same term god used on Abraham before :
Now The Lord said to Abram, "Go from your country and your kindred and your father's house to the land that I will show you.
And I will make of you a great nation, and I will bless you, and make your name great, so that you will be a blessing
. I will bless those who bless you, and him who curses you I will curse; and by you all the families of the earth shall bless themselves." (Genesis 12:1-3 RSV)
Another discription falls into Ishmael too:
"But God said to Abraham, "Be not displeased because of the lad and because of your slave woman; whatever Sarah says to you, do as she tells you, for through Isaac shall your descendants be named. And I will make a nation of the son of the slave woman also, because he is your offspring." (Genesis 21:12-13 RSV)
This is within the same meaning as above God hears Abraham's request with Ishmael because he is simply the offspring of Abraham
Genesis 18:11-12 Now Abraham and Sarah were old, well advanced in age; and Sarah had passed the age of childbearing. 12 Therefore Sarah laughed within herself, saying, “After I have grown old, shall I have pleasure, my lord being old also?”
The text never said anything about a plan , this is your personal interpritation. As I have showed above God still according to the biblical writers still told that he has heard Abraham
However there is a very big important point which I need to clarify here :
The original King James bible DOES NOT have the word "No" in it the text is simply :
And God said, Sarah thy wife shall bear thee a son indeed; and thou shalt call his name Isaac: and I will establish my covenant with him for an everlasting covenant, and with his seed after him (GENESIS 17:19)
What is really ironic too is that in the new international version you got the word "Yes" instead of "No"
Then God said, "Yes, but your wife Sarah will bear you a son, and you will call him Isaac. I will establish my covenant with him as an everlasting covenant for his descendants after him.GENESIS 17:19
New International version
You could check for your self :
http://biblehub.com/genesis/17-19.htm
So your whole argument which was based on No cannot have an accurate basis