"I shall make you into nations." Bearing in mind that Yishmael was already born, which "nations" was Hashem referring to?
Rashi: Yisrael and Edom (Eisav). 1
Ramban: He was referring to Yisrael themselves, who, on a number of occasions are called "Amim" and "Goyim."
Radak: It is Yitzchak, Yishmael and B'nei Keturah. 2
What is special about the fact that kings will descend from Avraham?
Targum Yonasan: Kings will come from him who will rule over other nations.
QUESTIONS ON RASHI
Rashi writes: " ... Yisrael and Edom." Ramban asks
Gur Aryeh: Because Hashem wanted to mention Yisrael, He also informed Avraham of all the nations that would descend from him. The next verse, "your children after you" (17:7), will single out Yisrael as the perpetuation of the Bris.