Couple of random thoughts:
Though it's definitely weird that there's no mention of Shaw, I am beyond excited about this, for two reasons. One: Shaw was–let's face it–not exactly the most interesting protagonist. David, on the other hand, was by FAR the most interesting character in the first film. Putting him front and center–at least in terms of returning characters from the first film–is a smart choice; it shows that Scott and team are genuinely taking the time to learn what worked best with the first film, then making changes accordingly.
The OTHER reason I'm excited about there being zero mention of Shaw is, as Bat Chain Puller alluded to in his 'Luke Skywalker Force Awakens' comment, it leaves open an interesting plot thread/mystery that Scott and team could pick up on later on, potentially in Film #3: what happened to Shaw? What exactly happened between the events of Prometheus and Alien: Covenant? Not only could this potentially create a nice cyclical, come-full-circle structure of this 'prequel trilogy,' but it also, in turn, could help to make Shaw a more interesting character in her own right. The Shaw that we had in Prometheus wasn't that interesting; a Shaw that's disappeared, whereabouts unknown, that shows up later on, having undergone who knows what kinds of changes? THAT'S interesting.
I also have to say: it's interesting that Twentieth Century Fox, by releasing this synopsis, has officially nailed down a direction-of-sorts for the franchise for the next few years, at least in terms of the films. In every interview that Scott's given over the last few months, it seemed as if he was throwing everything that came to the top of his mind out there; it was interesting stuff, don't get me wrong, but it gave off the illusion that he was sort of making this stuff up as he went along (the fact that the film's title seemed to be changing every time he spoke didn't help, either). By officially labeling Alien: Covenant as the "second film" in a "prequel trilogy," Fox seems to be making a distinct effort to simultaneously reel Scott's plans in (i.e. now it's a prequel trilogy, as opposed to three or four sequels) while also giving the director a clear set of boundaries to work within. As far as I see it, this is awesome news all the way around. I couldn't be more psyched