"I shall make you into nations." Bearing in mind that Yishmael was already born, which "nations" was HaSh-m referring to?
Rashi: Yisrael and Edom (Esav). 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 Bnei Keturah. 2
Gur Aryeh (citing Mizrachi): Yisrael alone comprises one nation, and this phrase is plural. However, refer to 17:6:1.1.
Perhaps Radak is bothered by the same question as Ramban - why does Rashi mention Esav, who would not fulfill Milah? Radak therefore mentions those nations descended from Avraham who do circumcise their children.
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 - Why mention Esav here, in the Parshah of Bris Milah? Esav, and the nation he founded, would not carry on the Mitzvah of Bris Milah!
Gur Aryeh: Because HaSh-m 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.