As something of an Alien fangirl 3 and 4 are more failures from a production standpoint than from a creative one. If you look at all the rejected pitches for Alien 3 there's a lot of interesting ideas which were never explored and a lot of studio fuckery in the final cut. I don't think the Fincher cut is amazing but I think it proves that there could be another excellent Alien movie.

Alien: Resurrection is plotted terribly and has all the shitty Joss Whedonisms you expect, but there's something undeniable about Winona, Sigourney and Ron Perlman in the grungy space aesthetic. The idea of Ripley as an Alien hybrid who is simultaneously attached to and repulsed by the Xenomorphs is interesting. Unfortunately they saddled the movie with a French director who couldn't speak English (and Joss Whedon, who arguably shouldn't speak at all).

The Terminator franchise is definitely more boxed-in: there's a core narrative about an important person who changes the world. Everyone else lives in contemporary LA, and the post-apocalyptic future is pretty boring.

The Animatrix anthology shows you can do lots in the Matrix world without needing the core characters. The themes and world-building could support a show like Cyberpunk: Edgerunners which is only tangentially related to the movies.