The Game of Thrones Season 6 finale, "The Winds of Winter," was predominantly Cersei Lannister's show, and there's never been a better time to look forward to Cersei's future by looking back at her past.
Back in the mists of time — well, Game of Thrones Season 5, Episode 1 — callous teen Cersei Lannister visited fortune teller Maggy the Frog, who lived in the wood near Casterly Rock. In a flashback in that episode ("The Wars to Come") Cersei demanded to know her future — and Maggy's reply is vital to understanding Cersei's fate.
If we break down Maggy's prophecy more closely, we can see that the first two predictions the crone made have come to pass — and the third tells us, ultimately, what Cersei's fate will be.
1. You will wed the king
Queen you shall be...until there comes another, younger and more beautiful, to cast you down and take all that you hold dear. — Maggy
Cersei married Robert Baratheon and was queen. It seemed that Margaery, younger and (according to her fans in King's Landing, at least) more beautiful, would be the one to take everything from her. In a way, she did: Margaery wrested Tommen's affections from his mother while everyone around Cersei left or betrayed her. However, with Margaery dead and Cersei sitting once again on the Iron Throne, it looks like Dany (or Sansa) will cast her down this time.
2. Will the king and I have children?
Oh, aye. Six-and-ten for him, and three for you. Gold shall be their crowns and gold their shrouds. — Maggy
Robert had plenty of baseborn children, but none in wedlock. Cersei was far too busy boning her brother to beget any legitimate heirs by a Baratheon, but she she did have three children of her own: Joffrey, Myrcella, Tommen. They were all golden of hair (and/or crowned with actual gold befitting their rank) and they're all dead, even poor, sweet little Tommen.
3. The Valonqar
And when your tears have drowned you, the valonqar shall wrap his hands about your pale white throat and choke the life from you. — Maggy
The most interesting part of the prophecy is "the valonqar." This is a term Cersei and others often use — disparagingly — for Tyrion, but the High Valyrian word means nothing more than "little brother." Tyrion is Cersei's younger brother, but so is Jaime, albeit only by a few minutes. Obviously, Tyrion has pledged his loyalty to Dany and has proved himself capable of kingslaying, so it could be him that chokes Cersei to death.
However, knowing George R.R. Martin and the exquisite agony and moral grey areas he loves to dance in, my money is on Jaime having to kill Cersei to stop her doing more terrible things, just like he did with the Mad King. Cersei's already gone full Aerys, and from the look of Jaime's face when he sees Cersei on the Iron Throne, he's already deeply worried about her reign. Of course, "Valonqar" could be a more metaphorical term, just as Myrcella had golden hair rather than a crown of gold.