Actually there's two possible scenarios I can think of where it might be OK to see the Jockeys back in another Alien movie, that wouldn't necessarily ruin it much. At least not for me.
1. In a prequel, that doesn't focus on them in the plot of the movie, but rather ends with them and doesn't show much more than what we already know. Maybe the Jockey ship flying through space in the end, with some brief shots of a non-fossilised pilot in his control chair, looking alive but still not being overly "animated". Kind of a "you know what's going to happen next"-ending. Although that could potentially be very cheesy too, depending. Could easily end up in more of a "I know where you crashed last summer" sort of feel, too.
2. A, either sequel or prequel (doesn't matter really), that shows more of the jockey culture without having any live pilot creature to further illustrate it. Like, more ruins in the same style as the derelict ship, maybe another dead guy, whatever. If it was portrayed as more of a long extinct civilization, showing a few more glimpses for the fans, but not really adding anything exceptionally different. I think if they stay clear of showing them walking around and being very animated, they wouldn't hurt the myth much.
Of course, most people might not care to just see more of what they've already seen in a sense, but, I personally would love to. I still love watching those scenes of the derelict interior in Alien, but I have to admit the same angles, same lighting, same fossil etc is getting a bit old.
