There are a lot of hints that the future games will focuse on Morrigan's child (probably not DA2 since the ckid will be less than ten years even at the end of the game but maybe in DA3 or 4) Now this could turn problematic because a lot of people didn't agree to do the ritual with her or didn't take her to the tower even if they did the ritual, but I don't think that would really have stopped her.
Now, even if you and Alistair/Loghain refuse to do it with her there is still a third warden, Riordan. Morrigan could probably have convinced him that they needed another person who could kill the Archdemon or just seduced him without mentioning the ritual.
Now when she has been impregnated she has to get to the top of Fort drakon unseen. Now since she is a shapeshifter, this isn't really all that hard. She even mentions turning into a bird in a conversation with Shale and if she can turn to a swarm of insects she can probably turn into a single insect too. And that's not even mentioning all other forms she could have without mentioning us. So she flies up to Fort Drakon and waits until you or Alistair or Loghain kills the Archdemon. Now she uses her magic to kill the warden slaying the Archdemon in a way that you can't see he/she was killed by someone else and absorbing the soul into the child. She then flies away while everyone thinks Urthemiel is gone for real. She has now gotten what she wanted while fooling everyone at the same time.
So what do you think about it? Does it sound plausible?
EDIT: something I forgot to add
You never see the warden who slays the archdemon die (and there is even a
giant flash effect to prevent the ingame onlookers from seeing
anything) so it's possible that Morrigan killed the warden with a spell
to make it look like nothing was out of place.
Edited by druplesnubb, 19 July 2010 - 07:13 PM.