1. Who will kill our Queen?
So, who will kill Cersei Lannister? We all know she's doomed to die, but by who? Let's see who has motives:
Arya Stark seems to be in the running. Arya's list began with Cersei Lannister when she falsely accused Ned Stark of treason against the crown, which led to his death. Arya's third person on the kill list is Illyn Payne, by the way, and since we haven't seen him in a while, I think we should assume his death is on his way on the back of a direwolf (for Arya Stark theories check here). Also, Cersei had Lady, Sansa's direwolf, executed even though we all saw Joffrey try to kill Arya, and because Nymeria, Arya's direwolf wasn't found, Lady was killed in her stead.
[Also, a fun fact, in season 3 episode 6, Melisandre prophesies that Arya will kill again before they meet once more. Melisandre says, "I see a darkness in you. And in that darkness, eyes staring back at me: brown eyes, blue eyes, green eyes. Eyes you'll shut forever. We will meet again." We know that anything mentioned in Game of Thrones is important later for future events. Arya has killed Polliver (brown eyes), Meryn Trant (brown eyes), the Waif (blue eyes), Walder Frey (blue eyes) and Rorge (can't tell if he has brown or blue eyes). What you can take away from that is that both Cersei and Illyn Payne have green eyes, and Arya has yet to kill a green eyed person. Just something to think about.]
Daenerys Targaryen is an option to kill Cersei. Obviously because Cersei stands in the way for Daenerys to become Queen, but Tyrion has probably explained everything Cersei has done and Daenerys may give Cersei what she deserve in fire and blood.
Speaking of Hand of the Queen, Tyrion Lannister has as much cause to kill Cersei too. Other than the fact that Cersei has been throwing shade at Tyrion his whole life, there are some important quotes between these two inferring future death. In season 2, episode 8, Cersei thinks she has found Tyrion's whore, Shae. Instead, they bring out Ros, who coincidentally was Tyrion's whore in Winterfell. Tyrion plays the part well, tells Ros to be brave, promises to free her, and promises to never forget her. But the best part to takeaway from this was this masterful quote from Tyrion, "I will hurt you for this. A day will come when you think you are safe and happy, and your joy will turn to ashes in your mouth. And you will know the debt is paid." That seems to be a pretty meaningful threat.
Another, less important, quote from Tyrion is made in season 3 episode 1 when Cersei says, "You're a clever man, but not half as clever as you think you are," while he replies with "Still makes me more clever than you." That might not seem important, but Cersei wholeheartedly hates Tyrion. She hates him because he's smarter than she is, she hates him because he's wiser than she is, she hates him because he knows how to rule far better than she does... and the ever so sad but very real issue: Cersei hates him because he "killed" their mother, Joanna Lannister. She died in childbirth after giving birth to him and Cersei blames him for that. Jaime doesn't, "He didn't decide to kill her." Cersei: "Disease doesn't decide to kill you. All the same, you cut it out before it does" (season 4 episode 10). And of course there's this gem after Tyrion learns that Cersei is offering a reward to whoever brings her Tyrion's head, "She ought to offer her c***. Best part of her for the best part of me." Lookout Cersei, don't make Tyrion mad.
All the while of making it seem like Tyrion will kill her, I have a far different belief. I truly believe that our killer will actually be Jaime Lannister. I know what you guys are thinking, 'what? No! Jaime loves her! He wouldn't!' and my response is, you're right. Season 1 Jaime wouldn't, but look who has no children and no Sept of Baelor left that he so very much tried to protect, Season 7 Jaime. Jaime Lannister is known as the "Kingslayer" for stabbing the Mad King Aerys II in the back during the Sack of King's Landing.
In season 5 episode "The Wars to Come", we see a flashback of young Cersei Lannister visiting Maggy (woods witch) at her hut near Casterly Rock. Maggy let Cersei ask three questions. First, Tywin promised she would wed Crown Prince Rhaegar Targaryen and she wanted to know when she'd marry him. Maggy responded that she wouldn't marry "the prince", but she would marry "the king" (Robert Baratheon). Cersei used her second question to confirm that she would be queen someday and Maggy confirmed she would, but she would be cast down by another, younger and more beautiful queen who would take all she held - at the time, Cersei assumed that to be Margaery Tyrell, but we can assume Maggy meant Daenerys. The third question Cersei asked was if she and the king would have children. Maggy says the king will have 20 children, but Cersei will have only three children. Maggy told her, "three children that gold would be their crowns - and gold their burial shrouds," implying all of Cersei's children would die before she would. Maggy also prophesies that Cersei will die at the hands of the "valonqar", which is the word for "little brother" in High Valyrian.
This is the reason I believe it to be Jaime Lannister. It easily could be Tyrion, for he is technically the youngest of the three, but Jaime was born after Cersei. Jaime is younger than Cersei, even for being twins. Jaime loves Cersei, but after Cersei's destruction of the Great Sept of Baelor and the murdering of Mace Tyrell, Loras Tyrell, Margaery Tyrell, the High Sparrow, Kevan Lannister, Lancel Lannister, and many of the people in King's Landing, as well as Tommen's suicide, Jaime must do what he can to protect the remaining people of King's Landing. Just as he did when King Aerys II kept yelling, "Burn them all!" Cersei used that same wildfire that Aerys wanted to use and actually burnt many people in King's Landing. I think Jaime will come to terms that he must protect the realm by being the "Kingslayer" and the "Queenslayer".
2. Cersei's Alliances
Wrapping up this theory post, who is left for Cersei to align herself with? We've got Qyburn, the Mountain and Ilyn Payne in King's Landing left on her side. We've got Euron Greyjoy and the rest of the Iron Islanders, but I'm not so sure Euron really stands a chance in this battle, but we'll just assume he's on Cersei's side. Aaaannndd, that's it? I'm not sure Cersei has any true fighters left. I guess season 7 will tell us!
Thanks for reading, if you even made it all the way down here!
See ya later nerds!