When isn't all that important, except I'd prefer if it was in the future from now. What Alien5 need is something new. I don't know what that would be. Maybe something could be made by exploring the intelligence of the creatures a little deeper. Having them on Earth is fine, as long as it is future Earth. Don't want cheap thrills by suggesting this could happen today.
Weyland-Yutani company must be involved, they and the Aliens go hand in hand (more so than Ripley and the Aliens IMO). Also I dislike the kind of notions presented in A:R of deliberately impregnating people to breed Aliens. The company is greedy and will not hesitate to go for profit over lives if the circumstances puts them in that choice, but they are not supposed to be deliberate cold blooded killers. That would make them less interesting to me.