Rashi #1 (to 15:6, in his second explanation): Avraham did not request a sign for HaSh-m's promise. Rather, he asked in what merit his children would keep Eretz Yisrael, and HaSh-m answered, "in the merit of the Korbanos."

Rashi #2 (to 15:6): Refer to 15:6:0.3 for the approaches of the various commentaries.

Ramban (to 15:7): HaSh-m explicitly promised him children, but regarding the land He said only "I took you from Ur Kasdim in order to give the land to you." Avraham did not request a mere sign, rather, a promise to receive the land.

Seforno: He was concerned lest his children sin and not merit the land.

Oznayim la?Torah: Because the current Pasuk was said five years before the beginning of the Parshsh, ¹ and following Hashem?s response, he never queried Hashem again. ²

Ramban: HaSh-m replied that His promise was unconditional, it was a covenant that was destined to be carried out irrespective of his or his descendants' behavior, or even on the Kena'anim doing Teshuvah. ¹

Rashi (to 15:6, in his second explanation): They will keep Eretz Yisrael in the merit of Korbanos.

Maharal #1 (Gevuros Hashem, Ch. 8, p. 44): According to the Gemara (Megilah 31b), Avraham's real question was that perhaps his descendants would one day sin and be destroyed, as were the generations of the Flood and Dispersion. Hash-m answered that Yisrael would never be destroyed, due to the merit of the Korbanos. When individuals who are intrinsically flawed, commit a sin, Korbanos will not help them. But Yisrael is intrinsically pure and wholesome, and any sin is incidental and not of their essence, and they can atone for it through Korbanos (or nowadays, by learning the laws of Korbanos).

Maharal #2 (ibid. p. 46): Eretz Yisrael is holy and it expels the wicked (Vayikra 18: 24-28). Avraham afraid that this would happen to his descendants.

Maharal #3 (ibid. p. 47): One opinion in the Midrash explains that because almost all people have children, after Hash-m assured him of it, he had no questions. But the gift of the Land, which was unique, and Avraham was worried and requested a sign.

Maharal #4 (Derech Chayim 1:2, p. 27): Avodah, i.e. Korbanos, is one of the pillars that the world stands on. This is because a servant has no reality other than his service to his master; similarly, every aspect of Creation exists only inasmuch as it is used in the service of Hash-m. This was Avraham's question; if the world was created for His service, and any sinner who deviates from that service will fade from existence, how would his descendants endure? HaSh-m answered that the Korbanos, which are themselves Avodah, would atone for those sins.

Lev Eliyahu (Bereishis p. 107): Avraham's Midah was Chesed. He wanted it to be like an inheritance, whether or not [his descendants] would be worthy of it. ¹