Even if you haven’t read the books, show fans have at least heard of the Valonqar in relation to Cersei in Game of Thrones.
Maggy the Frog mentions it to her when she tells Cersei her future.The show’s version of this scene didn’t include the last thing she tells Cersei.

“And when your tears have drowned you, the Valonqar shall wrap his hands around your pale white throat and choke the life from you.”

Valonqar means little brother in High Valyrian.
In Cersei’s fevered brain, Tyrion is the only person who fits the prophecy. He’s both little and little, so he fits the prophecy very well. But it’s not him.
Whatever Tyrion’s future might be, there’s almost no chance that he’ll end up strangling his sister to death. For onr thing, he could never get close enough.
So who is the Valonqar? The prophecy says the Valonqar not your Valonqar, which means that Cersei is not related to her killer.
So if it’s not Jaime or Tyrion who is it?
Let’s discuss the other little brothers who could end Cersei.

SANDOR CLEGANE
Aka The Hound. Little brother to Ser Gregor Clegane, aka The Mountain, Cersei’s undead personal guard and assassin. The only way he will kill the Mad Queen is if she’s collateral damage in his showdown with his brother.
ARYA STARK

Aka No One. Is a little sister, but the most popular theory (and my favorite) is that she will wear Jaime’s face to kill Cersei and Possibly The Montain. They’re both on her list. Probability it’s Arya: 80 %.
THEON GREYJOY

Aka Reek. Little brother to Euron and Yara. We last saw him sailing to catch up with the Iron Fleet to save his sister. Let’s say he arrived at the iron islands and discovered that Euron isn’t there. would he head toward Essos to try to catch up with Euron? Would he head to King’s Landing to wait? Or would he round up all the Ironborn to fight at Winterfell? Unless he happens on Euron actually bedding Cersei, there’s no way he’s her killer.
Probability that he’s the Valonqar: 0%.
THE UNBORN LANNISTER

Aka Demon seed. Little sibling Cersei’s dead children. The prophecy is specific about strangulation. A newborn can’t strangle anyone…unless it has inhuman strength. Which means…holy wights, I don’t want to think about what that means. This possibility has horrific connotations. Probability it’s the baby: 20%
JON SNOW

Aka The Bastard of Winterfell, aka The King in the North. I call him Aejon Targaryen.
He’s also little brother to Rhaegar’s children with Elia Martell, who were slaughtered during Robert’s Rebellion. The Lannisters murdered his siblings and his father (figure), Ned Stark. The Little Brother no one expected.
Aejon hasn’t been speculated about much as the little brother, and in truth, it’s hard to believe that he would leave the North while the war against the dead is raging. But let’s speculate.
Suppose he’s there to negotiate for Sansa or Daenerys (if they’ve been taken prisoner). Or perhaps he goes south to beg for more men after overwhelming losses.
Does Aejon travel south after the dead take Winterfell and are about to overrun Westeros? Possible.
This twist would explain the recent pictures of Cersei and Aejon filming a scene together in Dubrovnik (King’sLanding). Unless Dan and Dave are trolling us, and that’s totally possible, because

similar photos appeared before season 7. Aejon’s appearance in King’s Landing points to a confrontation. And it has to be an angry one, because Cersei lied to him.

In these photos you can see both Dan and Dave (the creators and writers) on the set, close to the camera. As if they’re directing. They told Entertainment Weekly that they will be directing the last episode. So.
If these photos are of the last episode, holy Mother of God. Not only is the Mad Queen still alive, she’s still the Queen.

Jon Snow, on the other hand, looks stressed out and unkempt. That hair! He looks tired, bedraggled, like he’s at the end of his rope. He looks too tired to kill anyone.
It’s possible he travels south in a last-ditch effort to save whatever is left of Westeros.

But for Aejon to lose control to that extent, to put his hands on Cersei and choke the life out of her? If she smirks at him, I can see it. Probability he’s the Valonqar: 50%.
If Aejon is the Valonqar, this means that the most popular theory about Cersei’s death — that Arya kills the Queen while wearing Jaime’s face– won’t happen. Watching Arya end Cersei would be excellent, but watching Aejon do it? Awesome!
The vengeance of two houses at once.
The wight baby though.
Who do you think the Valonqar is?
Let me know what you think of this theory in the comments below.