The prophecy for her life that she heard as a child is a big hint to her eventual fate. In the foretelling of her life, she hears "And when your tears have drowned you, the valonqar shall wrap his hands about your pale white throat and choke the life from you." Valonqar means "little brother" in High Valyrian, and many have taken that to mean that either Jaime or Tyrion will be the one to end Cersei. However, this newest theory poses a different idea entirely.
a parallel.— elena the cersei lannister apologist™️ (@gameoflena) April 16, 2019
*insert sad wine drinking emoji* pic.twitter.com/NO6Or2GyYS
One Reddit user, Namez_Are_Hard, posed the idea that it may not be her literal brother, but the little brother of her children. "What if she’s pregnant with another son, and while giving birth to him, she dies. She is killed by the Valonqar. Nowhere in Mags prophecy does it say HER valonqar, just THE valonqar."
The child would be the younger brother of Cersei's three already-dead children, Joffrey, Myrcella, and Tommen, and many fans are wondering if it could potentially be that "little brother" that ends Cersei and not her younger brothers.
The theory is certainly interesting, and we'll be waiting to see how and if Cersei meets her fate.