Really the sky’s the limit for speculation, as Dan Fogelman dropped the photo without context and is probably chuckling and reading comments because fans have no idea what is actually going on.
Another plausible This Is Us theory is that this is a daydream and that Randall is imagining William being present for the birth of one of his daughters.
Whatever ends up happening, we’re sure it’ll end in some fans shedding a tear or two.
Season 2 featured scenes where William imagined himself being a part of Randall’s childhood, so it’s entirely possible the show shot some quick scenes like this that show Randall thinking back on the memories he wishes he shared with his biological father.