I remember Scott specifically saying we'll learn who's in the suit in the end, in some interview. Dunno if he'd actually put David in there, but I doubt we'll ever know anyway.
I don't think Scott's interested in doing another film that focuses on horror/suspense with lots of aliens. He was trying to slowly move away from that. He said he'd do like 5 more if he had the chance, probably focusing more on David/AI, before connecting to the original.
So I don't see him being interesting in doing a more straightforward final prequel, unless they allow him to explore new things a la Prometheus. I think that might be why they're taking so long to figure things out.