Thalassa, the eldest sister of Gamora.

Had she lived, Thalassa would've led an early assault on Thanos's compound. Eliminating the Black Order decades earlier and saving countless worlds and trillions of lives. She would bravely fall in battle against the Mad Titan, yet her death would spur the Nova Corp and Ravagers to join together along with many galaxies worth of soldiers to Avenge her death and those that were slaughtered in Thanos's cruel quest. His death branches the universe into a more peaceful timeline.