It is true, as you say, that God covenants with Abram (later called Abraham) and does not ask anything as part of the covenant making. However if you read verse 6, you will find that Abram had already "given" God something -- his trust.
Abram believed the LORD, and the LORD considered his response of faith as proof of genuine loyalty. (Gen 15:6 NET)
Most versions say the LORD "counted it to him as righteousness."
It is my understanding that whenever anyone believes in God and puts his or her complete trust in God's message, God responds with promises of eternal fellowship. God didn't need to ask Abram to do specific things or to respond to later requests; it was already in Abram's heart to seek God's will as mentor and ruler. When Abram later did what God asked him to, it was not with any thought of getting a benefit for doing so; it was because he knew God asked for something that would be good for Abram in the long run and would enhance his relationship with God.
Ishmael was a member of Abram's household, and considered part of the covenant, although God told Abraham that the most important part would be fulfilled through his later son, Isaac. There was no guarantee that every descendant of Abraham would have numerous descendants, just that Abraham himself would. Abraham pleaded with God that Ishmael would also have blessings, and indeed Ishmael's descendants were numerous. Having an ancestor in the covenant was no guarantee of having the full benefit of the covenant. Each person had to choose whether he or she would also put full trust in God as Abraham had done.