Another question, what theories do people have about who the "Younger" person in Cersei's prophecy is? Here's the relevant conversation from the flashback:
Cersei: I'm promised to the prince. When will we marry?
Maggy: You will Never wed the prince. You will wed the king.
Cersei: But I will be queen?
Maggy: Oh yes. You will be queen, for a time... then comes another, younger and more beautiful, to cast you down and take all you hold dear.
Cersei: Will the king and I have children?”
Maggy: No. The king will have twenty children, and you will have three.
Cersei: That doesn't make sense.
Maggy: Gold will be their crowns... and gold their shrouds. *Laughs*
It seems implicit that this younger person is or will be a queen - given the use of the word "another" seeming to refer to queen from the previous sentence. Which would narrow things down considerably (basically to either Margaery or Daenerys).
However, technically speaking the younger person isnt'
necessarily a queen, or even a woman. The only requirements strictly are that they are "more beautiful" than Cersei and that they will "cast her down" and "take all she holds dear".
So i think at this point we have to ask what did or does Cersei hold dear?
Joffery, Myrcella, and Tommen: I believe that Cersei did love her children and did hold them dear. However we now know the mechanism of all of their deaths, and it was different in each case. Joffery was killed by the Ollena Tyrell, who we can rule out since she's not younger than Cersei. Myrcella was killed by Ellaria Sand who may be about Cersei's age, and has crowned herself princess of Dorne. I don't think she's more beautiful either inside or out however, nor will she really be the one to take the Throne from Cersei. Tommen killed himself because of actions that Cersei took.
Jaime: Would it be accurate to say she holds Jaime dear...? Maybe. Certainly she would like to keep him with her by one means or another and would be enraged if he betrayed her.
Power / The Throne: After Tommen's death, Cersei I believe mostly cares about holding power in her own right. She's tried all her life to wield power by ruling through others and that led to the loss of all of her children and the near destruction of her house - she is now done with any subtlety. Power is what she holds most dear now.
Personally, I think that the younger person who will stop Cersei from being queen will have nothing to do with her children's deaths, even if when we first heard the prophecy that's what people expected it to be about (and what Cersei expected as well). So they will be someone who will take away Jaime and will cast her down from the Throne.
Here's who I think the top candidates are:
Daenerys - I mean it seems really obvious at this point that Daenerys would be the younger queen to Cast Cersei Down and to take the throne from her (TOO obvious?) She has both the means and desire to do so. It's possible she might "take away" Jaime if he turns due to influence from Tyrion (of course Jaime seems to despise Tyrion due to Tywin's death). But I don't know, it seems somewhat unlikely to me that Dany could trust the Kingslayer...?
Jaime - Could Jaime kill Cersei and fulfill the prophecy? He is a few hours younger than she, and it would be ironic if Jaime is the one to kill Cersei. Plus he has more of a means to do so currently than anyone else. If Jaime learns that Cersei essentially killed Tommen would that be enough to motivate him? What if she decides to blow up King's Landing rather than allow Daenerys to take it ala Aerys. Also Jaime himself need only take Cersei's throne... since he himself is all she holds dear other than that! It would also be awful and traumatic so I'd rather not see it.
Brienne - She is called "Brienne the Beauty" mockingly but if we take the statement metaphorically then Brienne is more beautiful than Cersei in many ways as well as younger, and there would be a nice irony there. Plus Brienne is much more likely than anyone to "take" Jaime from Cersei (meaning his loyalty and affections). But Brienne is not in a position to take from Cersei what she holds most dear - which is power. So I definitely rank her below Dany or Jaime.
Tyrion - Could Tyrion be described as "younger and more beautiful"? In the same way as Brienne, I suppose. He may cast Cersei down with Daenerys' help, and may influence Jaime to leave Cersei as well. But I don't know how we would be distinguishing Dany from Tyrion in this, unless Tyrion kills Cersei with his own hands.
Sansa - She used to be a dark horse favorite for this prophecy, and it would be satisfying to see Sansa get some payback against one of her early abusers, but it seems pretty unlikely at this point. After last season the Queen in the North ship has sailed. And I don't think Sansa will be going South, nor does she have any likelihood of stealing away either the Throne or Jaime. Perhaps Jaime will go north to finally fulfill the vow to Catelyn but he'd be going not just for the Starks but also because Brienne is there.
I think these people can be ruled out:
Margaery - In fact, Margaery never "took" anything from Cersei, not really. She did influence her sons, but ultimately Cersei held power to the end, and her sons'
lives were taken away by others. Margaery never held any real power in King's Landing - even if she had the common people on her side. Also she had nothing to do with Jaime.
Stannis - This was based on the Valonqar (younger brother) part that was omitted from the show. As we know from S5 Stannis got no closer to KL than Blackwater Bay and he's now rotting somewhere in a field outside Winterfell so... I think we can rule him out pretty definitively. :p
The Hound - "Cleganebowl" theory that the hound would defeat The Mountain in a trial by combat for Cersei, thus condeming her to death was rendered null and void by S6E10. There will be no such trial for Cersei. Also I would not personally describe the Hound as more beautiful than Cersei... Nor is he younger IIRC.
By the way
here's a relevant essay from watchers with some evidence for each candidate (some of which we know are wrong now)
ETA: I forgot to mention one other strong candidate!
Arya - Arya is actually in Westeros now, and has the means to kill Cersei since she has FM training. And Cersei is on her list. She's certainly younger and maybe more beautiful (Maisie is adorbs). Anyway I think Arya has a better chance than Sansa now of killing Cersei. In this theory I suppose Arya would kill Jaime and then Cersei, thus taking all she holds dear... Yikes. But then again Jaime is not on her list and she has already passed up one opportunity to kill him.