Sigourney Weaver was recently honored at the Hero Complex Film Festival where she discussed the possibility of another Alien movie.
Among other things, she mentions she didn't want to film two Alien movies back-to-back, and she also says she wasn't thrilled with the idea of bringing the Aliens to Earth, which is where she suspected it was heading to, and of which Joss Whedon wrote the script for.
Here's the blurb (via Collider):
I feel after going to a couple of these Comic Cons and meeting so many fans who are so passionate about the series, passionate about Ripley — that there’s more story to tell; but I don’t know how to do that. I don’t think Alien belongs on Earth popping out of a haystack, which is where I was afraid it was going to go. I feel it should take place in the far reaches of the universe where no one in their right mind would go. There are very few filmmakers that I can think of that I would want to entrust this to. But I can think of a couple… I feel there’s a longing in certain groups of fans when I meet them for the story to be finished because we really left it up in the air and I feel a bit badly about that because I was part of that decision making process. I didn’t want to make four and five in a bundle. I think it’s hard to make these films all in a big lump. You need time to sort of let things resonate so I can imagine a situation where we could at least finish telling her story. I think that would be very satisfying at least to me — although I haven’t done anything about it; but I can understand why that could happen and I certainly know young filmmakers who are interested in doing that. So we’ll just have to see what happens.