Ok, so even though I can appreciate that the dark ritual represented only 25% of endings for Origins, I would just like to point out the importance of having a child with the soul of an old god. I mean, we are talking about the most powerful beings in existence, so powerful they turned humanity away from the maker (according to the Andrastian Chantry at least). All of the uncorrupted Old Gods are still slumbering in their prisons; having one loose in the world will make the mage/templar conflict seem like a job board quest by comparison.
We don't really know to what extent the being will have the personality of Urthemiel. The personality of the child may come through, but does it really stand a chance against an Old God? This could lead to some interesting moral issues regarding duality, which is quite an important theme in Dragon Age; you have the duality of the Elven Gods with the Forgotten Ones, the duality of the all-sacrificing Andraste with the jealous Maferath, the duality of ultimate oppression in the templars compared with the freedom to command kingdoms as a Grey Warden. The Old God Baby could represent the culmination of this duality; a normal (well, ish, considering it will be raised by Morrigan) child, compared to an ancient entity, which won't be best pleased about the way the Maker put it to sleep. Perhaps we'll get an Anders-Vengeance thing going on?
In short, even if a mere 25% of us chose the Dark Ritual; and to be honest I suspect far more did so, since this allows your Warden to go down in history as one of the few who slew an archdemon, as well as being the only being in existence to survive doing so. (Hawke is the most important character in Thedas my left bum cheek) As such, it makes sense for Bioware to do more than a series of side quests regarding the OGB; at least I'd expect a companion, or perhaps an Awakening sized expansion. And to assuage those who didn't perform the Dark Ritual, said expansion, could quite easily include an alternative beginning, where Morrigan found an alteranitve way to perform the ritual. No one knows what secrets were kept in Flemeth's Grimoire; for all we know, Morrigna sleeping with the Warden was merely the easiest way for her to acquire the soul of Urthemiel. It is quite possible, perhaps with the advanced blood magic she shows in fixing the Eluvian, that she took the soul of the Archdemon without your knowledge. Indeed, the essence to power such a spell could come from that which would have been saved by your Grey Warden living; instead, your incredibly powerful tainted blood is used to power the spell.