Lindelof's comments at the end pretty much confirm for me we're going to see a Xeno at the end of Prometheus. What else could he mean by comparing it to the encore at the end of a rock concert? Think about the implications of that comment - when a classic pop/rock act play one of their most famous numbers as an encore, that means the fans get to see/listen to/experience something from the past that they all love. SUREly that's the Xeno?
Plus I've always felt, since there's been this emphasis on the "final 8 minutes" or whatever, that they simply wouldn't be able to resist a Xeno moment. I know Prometheus is a very different film to a direct "prequel", but I've always felt the introduction of the Xeno, with its iconic status, is simply too powerful an image to overlook.