It's never going to happen.
Almost certainly true.
Bearing in mind she's 64 years old now, time is ticking.
She looks great for her age though, I have no doubt she could handle it like the rest of her male brethren from that same era are now doing. The weakest link in most of their outings has been shoddy wink-wink writing that can't seem to find the earnest sensibilities of the golden age.
The real question is... It's been twenty years since A:R. What exactly do you do with an ALIEN 5? Personally i'm not all that interested in seeing the continuation of Ripley 8 unless we're going to get some really Earth-shattering writing and directing.
Alien 5 really should deliver on the unkept promise of the Alien 3 teaser...
I don't want to hear "AVP" or "AVP:R" as responses. No. That's not even remotely the same. Not this kind of execution. I'm talking Cthulhu, Hell mouth's opening in the Earth, people taken in the night. Tap those deep dark nightmares, violate our personal space, scare the Hell out of us, and give us a damn good heroine. If there was ANYTHING truly missing from Alien Resurrection above anything else, it was the Ellen Ripley that really cared about people. I want that Ripley brought back. Make that the story arc for her amidst a genuine fight for survival on Earth. There is nowhere to escape to. They're here, on Earth. It's fight or die. Instead of showing someone losing their humanity in that, have Ripley find hers, and teach the survivors how to deal with the aliens through her connection to them.
Or as someone else said, reboot. Though i'm honestly not keen on the idea of a reboot. You could just as easily sidestep the issue and not ever go back to Ripley ever again and just tell a new story. As the comics and novels have shown us there are many stories to tell without Ripley. Granted not all of them are the stuff of greatness, but there are diamonds in the rough.