This is actually very simple - if you read the prophecy without making the same mistakes Cersei does, there's only one person who fits what it actually says.
That one person sounds impossible - until you think about it, and realise it's exactly the sort of twist GRRM would do.
GRRM's characters are always misinterpreting prophecies, letting their biases creep in and paying attention to the wrong things. In particular, Cersei (and most readers):
- Takes the "valonqar" section out of context and fixates on it.
- Gets distracted by the evocative language.
So let's look at the whole prophecy...
Six-and-ten [children] for him, and three for you. Gold shall be their crowns and gold their shrouds, and when your tears have drowned you, the valonqar shall wrap his hands about your pale white throat and choke the life from you
...then let's translate it into plain English, so we don't also get distracted by its flowery frills.
The king will have 16 children, you'll have three. You'll see your children crowned, and you'll see them die, and then, when you are mourning, the younger brother will kill you by strangulation.
(it's probably fair to assume that "Gold shall be their crowns..." and beyond refer to her three only, not including the king's sixteen, because many have already died without gold being their crowns)
Note it says "the valonqar", meaning "the younger brother". Not "your younger brother" or "a younger brother", but the little brother already established by context.
Cersei immediately assumes it's centred around her. Even at this young age, she already hates her younger brother Tyrion, and leaps at the chance to hate him more. Having come to this conclusion, she treats "The Valonqar" as some kind of dread title for her "monster" of a younger brother.
But it's not. It's just a foreign-language way of saying "the younger brother".
It's Tommen, obviously.
The younger brother among the three siblings who were just introduced and are discussed in that very same sentence.
If someone says, "Can you look after my three children this weekend? We're going away. The younger brother is allergic to peanuts", you wouldn't think "Does she mean my younger brother? Or maybe she means the younger brother of my father's henchman?".
It's disguised by the evocative language, and by Cersei's character weaknesses (particularly her prejudices against Tyrion and her inclination to make everything about herself). This is exactly how GRRM does prophecies.
But isn't that impossible?
For it to be Tommen, three things would have to happen:
- Tommen would already have died - so would have to come back from the dead, somehow.
- Tommen would have to transform from a soft little boy to something strong enough to strangle a reasonably tough fully grown woman.
- Tommen would have to transform from a gentle, mild-mannered boy, into something aggressive enough that it would kill its own mother, and would do so by strangulation.
Surely such things are impossible in the world of ASOIAF?
How could something as devilish as someone coming back from the dead possibly happen in King's Landing?
And would GRRM really have Cersei die at the hands of her own gentle, beloved son, while she was literally still mourning his death? Especially since, going by trajectory, it looks like GRRM is going to spend the next book trying to lead us towards almost feeling sympathy for the newly-humbled Cersei, as she, presumably, tries and fails to protect her children while surrounded by manipulations and provocations she doesn't understand from Varys and others.
Presumably, it would be a chapter from Cersei's point of view. We'd see the full, crushing grief as Cersei "drowns in tears" and finally snaps, having now lost everything that made her life worth living - first her ambitions, now her children.
We'd see her lose the last of her sanity. As zombie-Tommen approaches her, we'd know what was happening, but she'd probably misinterpret it, possibly thinking some miracle had happened. She'd rush to embrace him, deluded - and then she'd feel cold hands around her neck, and the chapter would just end.
It'd be a chilling mirroring and twist on how Cat lost her mind when she lost everything she'd been holding herself together for at the end of her final chapter.
GRRM wouldn't do that to us, would he?
p.s. I'm avoiding reading Winds of Winter extracts until the book is released, so if there's anything in one related to my answer, please don't tell me about it - thanks!
Additional small twist. If the above is true, it gives a clue about something else likely to happen:
If Cersei dies shortly after Tommen does, it stands to reason that Myrcella should already have had her golden crown and golden shroud. So it sounds like the plot to crown her in Dorne might make a resurgence - maybe after news arrives of Quentin's deep-frying. Either that, or Myrcella has a very fast and (literally) short-lived coronation in KL.