Playing through the demo again, and watching the Flemeth scene...I'm almost convinced she's an Old God. If it was Andraste, why did she teach Morrigan the spell to preserve the soul of an Old God, and why was she so eager to stop the Blight? Unless she's an Old God, and she wants to free the ones that are left. The only way to do that is to stop the Blight. She probably figures even if Morrigan has the Old God child, he'll turn to her side simply because he's an Old God like Flemeth.
She saves Hawke because she sees that he/she's going to cause strife in the Chantry and rock its power no matter what- which, if you were an Old God who got struck down by the Maker, you would want. A perfect time to strike. And then the Warden and Hawke both disappear- the Warden knows about the DR, Hawke knows about the amulet. Flemeth could have done it, or maybe they're working together (for or against Flemeth, I suppose)
But there will be variables in her plot- can Morrigan actually stop her? If you did the Dark Ritual, and you find the child later (maybe grown up, heck, he could be the PC like the rumor was this time), could he decide not to follow Flemeth? And if you, say, side with the Maker, is it necessarily the right thing to do?
(I need to speculate to keep the mind busy till midnight, haha)