While I doubt that the inquisitor is the Child of Morrigan, I'm almost certain that she--and the OGB--are going to show up specifically BECAUSE of the Inquisitor's ability. Nobody really seems to know what Morrigan wanted with the baby; it's quite obvious that saving the Warden or Alistair wasn't her only motivation, as she'll mention the dark ritual regardless of player approval. Morrigan wants power--or maybe it's even, at this point, Flemeth who wants that power. We can't say for certain.
So, why would a witch with the power of an old god at her disposal want a being with the power to enter the fade at will, and possibly open and close tears into it? That's the real question, I think. Personally, I believe that Flemeth--and possibly Morrigan's goal now--are to re-storm the Black City, with the power of a cleansed old god at their disposal. Whether their intent is to gain the power of a true god, one with mastery over the fade, or to cleanse it, is up for speculation, as is this crazy fan theory I've cooked up here.
EDIT: In playthroughs where Morrigan doesn't have the old god baby, finding a way to harness the power of the fade through some cooked up ritual would be an incredible power in your arsenal.