You know, I'm aware that not many people on this forum like Fable, but I DO like how they addressed 'cannon' in Fable 2. There method of storytelling allowed a senario where there was no cannon for Fable 1, so players of Fable 2 didn't have to live by someone else's version of events.
First off, they made Fable 2 take place 500 after the first game and burned the Hero's Guild to the ground. This allowed Lionhead to create a world free of cannon. This is because everything regarding the Hero of Oakvale burned with the guild, leaving only rumors and legends behind. Some residents of Albion believed the hero to be benevolent, others evil. It was up to the player to decide which was true.
BioWare could easily copy this method of storytelling by making the stage of Dragon Age 2 take place in a different country hundreds of years after the Blight. Burn down an important library in a revolt in Fereldan, and you've effectively created a senario where the new hero could have been told that the last Warden to defeat the Blight was an elf. Or a human noble. Or a Mabari hound.
I'm not suggesting the senario would exactly play out like that, but something similar, and perhaps better executed, would certainly work towards the benifit of all without inserting cannon into anything.
Secondly, no characters in the sequal, save Theresa, had anything to do with the original game. This stopped the player from asking too many how, what, where, when, why questions because the game, for the most part, had a completely revamped cast, and Theresa's past was left so ambiguous that it didn't conflict with the previous Fable.
I do have an inkling that we may not have seen the last of Morrigan or her 'brood'. Even if the Warden turns down her ritual, there's nothing stopping her from having children. I don't know how birth control works in Ferelden, but to my knowledge if she gets jiggy with someone, she stands a good chance of becoming preggers and furthering her line.
And let's say her and this child (soul of an Old God or no) do appear in the sequel. Morrigan is so tight-lipped, I doubt she'd say anything about her time with the Wardens, especially whether or not she had relashions with one to create a god baby. Morrigan is all about survival and secrets, so this doesn't strike me as information she would willingly part with, so once again we're left with a situation where the player can assume whatever they like.
Really, even if you're writing Dragon Age novels and comics, you don't need 'cannon' to make it work. There's always a way around the 'details'.