At one point in Uncharted 4 (which by the way is amazing) - I'm not going to say where, in order to avoid spoilers - you can find this poster.
Here you can see what looks like Ellie wearing a gas mask, and pregnant.
Now some may ask, is that even Ellie in the first place? Why would she be wearing a gas mask if she's immune? Well she's also pregnant in the picture, so she's probably wearing that gas mask so she doesn't risk her unborn child being infected, because she would have no way of knowing if her baby could be immune or not. I could be wrong, but I think it's a safe bet to say that the woman in that picture is a pregnant Ellie.
Why this is totally Ellie
First let's compare it to other pictures of Ellie (there are plenty in this article). I've already touche on how her being pregnant, could be the reason she's wearing a gas mask. The girl in the picture also has the same style of clothing as Ellie (she does were different outfits throughout the game), and she has the same hair color and style as Ellie. Not to mention she even has dirt in the exact same places as you usually see on Ellie.
One big thing people like to point out, is that the women in the photo is holding a gun in her left hand, and Ellie is right handed. However, take a look at the cover of The Last Of Us Remastered,
Hear you can see Ellie (and that IS Ellie) holding a rifle, left handed. Also note that Joel is right handed as well, and in the original cover of the game,
Joel (and that IS Joel) is holding a gun in his left hand. Also note that it is the same type of gun Ellie is holding in the poster from Uncharted 4.
Personally, I really like this idea. Ellie has kids of her own, the game could revolve around her protecting her kids (most likely at leas one daughter, hence the "American Daughters" title), as Joel once protected her.