The sequel needs to begin with us finding out Weyland had a second team following him to LV-223 to pick up his escape module, the new team sets down and discovers the dead engineer and the dead big face hugger and no survivors, then we have a nice classic alien sequence with the deacon attacking this new crew...after they kill the deacon (first 3rd of the movie) they go and explore the caves, they discover a message Shaw left, that lets them know where she was heading, so then they take off after her...we don't see Shaw until at least halfway through the film...then we follow Shaw and David as they reach the Engineer homeworld...they set down...(pro writer fills in this part) but the basic idea is Shaw discovers something entirely unforeseen about the Engineers...maybe she discovers they are breeding Xenos or whatever...but shit goes south in a hurry and she is taken by the engineers...then the new crew shows up and has to rescue her...but bottom line there needs to be a second crew, if Shaw and David are the only human/oid characters I don't think it would work