Ridley wanted to do Prometheus without aliens at all, and he definitely wants none in the sequel.
Of course, seeing the concept art for Prometheus, this shouldn't have been a problem. But the film was kept so short, with so little of the concept art turned into reality. Ridley "wanted to keep things real" and as a result, there's almost nothing. Feels kind of alien-lite, alien in the sense of any kind of extraterrestrial life. The thing is is that Alien and its cohorts are, at their cores, monster movies. It can be a guy in a suit or a puppet, but if you remove it, something is going to feel missing.