Here's a new interview with Neil Druckmann and Faith Erin Hicks (who drew the comic). It's filled with Left Behind spoilers - in-depth plot discussion, so watch out. The usual interesting interview from Druckmann where he talks about the writing process. This paragraph is of note;

I still have this script written, this story about Ellie’s mom [for] an animated short we were going to do, but it fell through. I’d love for that to see the light of day sometime, maybe as a DLC or a comic book. We’re brainstorming the next [Naughty Dog] project right now; some of the ideas are sequel ideas and some are brand-new IP. We’re just trying to see where our passions lie. Is there more to do here [in The Last of Us] both on a mechanical level or a narrative level, so we’re not repeating ourselves? Or is it this a good point to say goodbye to the characters in this world? It’s a very heavy decision, because whichever direction we go in commits us for the next three to four years. So we’re going to take several weeks to make that decision. Even if we do go to new IP, I’d still like an opportunity to revisit these characters and I’d love to have Faith come back and work with her again.

Sound's interesting. As with their sequel talks, it's all very "ehhhh we'll see", but interesting regardless. If you missed it; all talk of Ellie's mom is very weird and deliberately ambiguous. In her letter to Ellie she talks about knowing her life is cut short or something, and that Ellie will learn something about her soon, and tells her "life is worth living." The comic and DLC tell us nothing about her, despite Marlene seemingly being her best friend and Riley knowing Marlene well.

I wonder if the sales of Left Behind could do well enough that they may consider at least developing one more little DLC story... Unlikely; a new comic book would be the best bet, I think. Even though he's acknowledging the idea, I wouldn't imagine they'd be able to go through with it, or at least couldn't match the production values of Left Behind. Always wished we could have seen more from the perspective of Marlene (which could possibly be set during when Ellie's mum was alive) or Tommy in-game, too.

Here's a better idea. TLOU 2 exclusively on the PS4.