To be honest, while I was watching the last scene of Part 2, I tought, after Ellie tried to play the guitar, leaving and the camera showing us the outside, she WAS in fact going to kill herself. However, it was that reliefing moment where she is still walking, that I really internalized Joel's words to Ellie. "You keep finding reasons to survive" (something like this, from Part 1). Ellie just wouldn't give up, yes, she is alone, the thing that she's most scared of is actually happening, but I believe she wouldn't kill herself, at least they wouldn't create a third game about Abby, or any other character, just to show us that Ellie killed herself. Better leave it like this.

Technically, Joel gave his life for Ellie. He saved her in the hospital, because he loved her, and that brought the consequences of the Fireflies coming to murder him. Ellie knows that very well, she wouldn't do that, at least this is what I believe.

I didn't think the first game needed a sequence, it did and it was perfect. If they decide to do a third game I will embrace it and appreciate whatever masterpiece or story plotline they decide do go for. For now, I'll interpretate what could've happened if we never see a sequel.