I saw a pragmatic take in the comment section as a new definition for canon, as "What holds predictive power" over what we might see in a film or video game in the future. Especially where it might restrict what a future writer or director can do.
To me, that is really all that matters. Does it matter if Bishop Weyland in Alien-cubed was a human or a synthetic? Well, he was an android in the Colonial Marines video game, does that count? In so far as how that might restrict a future director? Highly unlikely. It won't matter.
Same thing for the events in comic books. Ideas might get taken from the old comics, but probably not more than cosmetic ideas like a director saying "oooh that looks neat, send that to the design team and tell them the monster in this scene ought to resemble that and come up with some ideas based on it." And this is in the unlikely event they even see something from the comics at all.
It's also possible that future directors will be allowed to disregard the events of movies that didn't do well in theatres. If you factor in the production cost, the marketing cost likely equaled, and the theatres taking their half, and the net is in the red, as in the movie lost money, then the studio execs could very well look at the whole thing like something they want the audience to forget going forward, and might even insist a director disregard the events of a film.
Basically, the only hard predictive rules are that we shouldn't expect something obscure from the Dark Horse EU or video games to show up, references to the events of movies that lost money, or anything from another IP to show up. As Alex White pointed out, they cannot reference anything having to do with predators when the license is only for Aliens.
So, what do I treat as "canon" for discussing what we might see in a movie? Basically, the events of Alien and Aliens.
I'm still allowed to hope egg morphing and alternate ways of alien reproduction, besides the queen, are shown.
Now that absolutely doesn't mean that we, as fans, shouldn't have fun speculating and discussing EU events in their own context. There is a lot of great stuff there for fun conversations. But we ought not pull hard rules from them and think they need to apply to anything a current author is doing.
We also shouldn't devalue something just because it doesn't hold predictive power either. Want to discuss the stuff in Aliens: Genocide? Cool. How about a video game you loved? We shouldn't dismiss a topic just because it's about something non-canon either. I mean, we could watch all that the studio sees as canon in under 24 hours, but it would take much longer to re-read through all the comics, novels, and replay all the games. So, being non-canon shouldn't mean less value to our entertainment or imagination or that it is somehow less valuable for discussion. Again, we just shouldn't think hard predictive rules can be made from it. In fact, in regards to the Alien itself, it's better that there are fewer hard rules and more freedom for creatives so that they stay unpredictable and mysterious.
