If you had told audiences that one of Thanos' daughters was going to be unequivocally murdered during Infinity War before its release, almost everyone would have put money on Nebula losing her head.
But in a truly devastating turn of events, Thanos instead killed Gamore on Vormir in order to get the Soul Stone. And though Nebula was held prisoner and tortured by Thanos in Infinity War, she eventually broke out and got to help the Avengers and Guardians on Titan try to stop him. They may have ultimately failed, but at the film's end, Nebula was still determined to see her abusive father die.
The trailer only features a couple of shots of Nebula but one of them crucially features the brightest of the aforementioned lens flare-inducing sunlight motifs. This and a few other teases from throughout the franchise's past are enough evidence to lead one to believe that Endgame may stay true to Thanos' defeat in the Infinity Gauntlet comics and have Nebula kill her own father.
Honestly, it feels like this is where it has to go. After being tortured and abused by him for her whole life and him killing Gamora, the only way for Nebula to really become a hero and free herself is to kill Thanos.