BROTHERHOOD OF STEEL QUEST 16 - A New Dawn
Triggered by completing Brotherhood of Steel quest 15 - The Nuclear Option
Objective: Return to the Prydwen
Fast travel back to the floating fortress and you'll find Kells and Maxson waiting for you in the control room. Maxson will confer the rank of Sentinel on you as a reward for all your efforts in aiding the Brotherhood. You also will obtain a jetpack mod for your power armour (woo!) and be pointed towards the Cambridge Police Station for additional sidequests you can take on at your leisure.
Congratulations, you've finished the Brotherhood of Steel questline and have brought the story to a close. However, there is still much of the Commonwealth to explore and multiple other story paths available.