I wonder if she takes off her helmet in the movie? If she does, will she be a CG alien or look like she does in GOT.
I've been wondering the same thing. One theory I had could be this...
In the now "gone but not forgotten" EU, there was a character named Winter who was a childhood friend of Leia and at one point, she was caring and providing protection for the children of Leia & Han.
So what if, in the years following RotJ, Han & Leia still did have fraternal twins, one a boy and one a girl. Leia and Han still very much in the thick of things dealing with the remnants of the Empire put the kids into hiding with a new version of the Winter character.
Then, at some point, someone on the First Order side of things, perhaps Andy Serkis' character, wants to get a hold of those kids having figured out the Darth Vader/Anakin Skywalker-to-Luke & Leia connection and an attack is launched to kidnap the kids and replace the kids with burnt corpses. This new version of Winter has to make a quick "in the moment" choice - stay with the kids to watch after them or go down fighting overwhelming odds and the First Order takes the kids anyway.
The First Order allows new Winter to continue to care for the kids, perhaps they even tell the children that their parents are dead and that the First Order is the group that "Saved" them. At some point, new Winter still trying to do the right thing, tries to get free with the kids but due to that attempt being thwarted only the daughter gets "away" which results in her becoming "lost"/"missing" - new Winter is tortured and scarred for letting the daughter escape and given the opportunity to "repent" by staying with the son as his protector.
Daughter is Rey, son is Kylo. Kylo grows up and puts his pseudo-Mom in charge of his troops because she has been this bad-ass female role-model he's looked up to for as long as he can remember.
So when Phasma finally takes off her helmet, Han and/or Leia are present and immediately realize that the woman they had trusted to protect the kids didn't die when the kids did AND that their kids are both alive and well.
While I know this reads like fan-fic b.s. - I think something along these lines could work and allow Phasma to have a "redemption" arc that is very prevalent in movies these days... It also still fits in with the idea of Leia & Han having twins, something that surely at SOME POINT, Lucas must have approved, so there's no reason to totally throw out all aspects of the post-RotJ EU, just the stuff that doesn't allow for this new flow of events to occur...
So my vote is that she will be just like in GOT, but with part of her face scarred - like from a force lightning strike.