Covenant could be just great, and I think that's going to be the case...
I personally never considered Prometheus a failure, but a mere drop in some aspects of writing and character development (other problems: too much ambiguity, and lack of psychological horror). No one can really say that main ideas in that movie were bad, on the contrary, ideas and motif (mythological, religious and philosophical) were great.. and so I hope that Ridley is going to fix mistakes from Prometheus (characters, story development and too much ambiguity) and give us a great story about Engineers, humans, androids and xenos... and maybe someone new..
If he fail to deliver a satisfying movie, then we can say goodbye to this franchise and to two other planed sequels (in fact prequels to Alien)...and that fact was surley in the minds of Ridley's and others from Fox... They know what this movie must be and what it doesnt... I myself would consider Covenant a failiure if they disregard all the story they told in Prometheus... This movie must in some extent explain the story of Engineers and explain major questions from Alien and Prometheus (not all, because mystery is essential): What part will humans and David play in Xeno and Engineer story, why they want to destroy humans, who created xeno and why, space jockey... I dont want full answers, but movie need to show us something that will lead us to some answers. They also need more psychological horror, mystery, great questions and a few answers (few big hints will sufice)... Prometheus failed there, because that movie just imposed number of questions without any concrete answers...
I m very optimistic about this one...
P.S. I'm satisfied with cast choices...