If James Cameron is to be believed, if there is a fifth Alien film, and if he is involved (something he is very interested in), then the film would likely be a bit of a 'reboot', and Weaver wouldn't be involved.
He states that some of the themes she was interested in exploring during Aliens, and that he disregarded, eventually showed up in the third and fourth movies (Ripley dying/having sex with the alien), and those aspects were part of the reason the franchise faltered.
And he is
very interested in getting involved in the Franchise again. He has gone on record as saying that an Alien vs Predator movie would kill the franchise altogether, and that occasionally he and Ridley Scott have discussed making a fifth movie together!
Anyhoo, I got this info from my Empire Magazine's interview with Cameron about his body of work - it's a very interesting read, and might be available at
www.empireonline.co.uk