Aqua could have changed his fate in Oshi no Ko by surviving the murder-suicide with his father, Hikaru Kamiki. Instead of dying with Kamiki, he could have gone into hiding, or made it out of Japan to avoid legal repercussions and continue his life. He also could have allowed his plan to succeed in a less fatal way, for example, by using the resulting scandal to jail Kamiki rather than resorting to a death pact.
To survive the confrontation with his father:
Flee Japan:
Aqua could have fled the country after his confrontation with Kamiki to avoid the investigation and punishment.
Go into hiding:
He could have gone into hiding within Japan, similar to the "Dark Knight Rises" ending, to escape the consequences of his actions.
Stage the incident differently:
Instead of a murder-suicide, he could have made it appear as though Kamiki was the sole instigator, allowing Kamiki to go to jail and Aqua to live.
How to achieve a less fatal outcome:
Allow Kamiki to go to jail:
Aqua could have presented enough evidence to the authorities to ensure Kamiki's arrest for Ai's murder, without needing to kill him in the process.
Cooperate with others:
Aqua could have continued to work with individuals like Kana or Taiki Gotanda to achieve a successful outcome for his revenge plot, rather than making a suicidal choice.
Prioritize his own well-being:
By choosing a path that prioritized his survival over the dramatic climax of his revenge, he could have lived to see another day