I think she gets badly injured protecting Joel. She has to be left behind and she either survives or dies. In the Pax demo, she saves Joel from and infected. Or Joel refuses to let her come any further with them because he loves her and doesn't want anything bad to happen to her, as she is the last thing he has left.
Either way, Tess is still a total badas s.
Tess is preggers and gives birth to Joel's child. Ellie is like ooh its so beautfiful cause shes like never seen **bleep** kids being popped out before.
Then she gets killed by the military people with shotguns and also kills Joel's baby by falling on top of it.
That would be the ideal situation.