Having completed some more of the DRK quests (and went to check out more info because I needed to know and leveling is taking forever lately), my money is on Rielle becoming Ratatoskr.

No real basis in logic beyond the obvious points gleaned from the DRK questline, but it makes a certain kind of sense. Rielle is a relatively minor character, so her becoming the "sacrifice" or what-have-you that's necessary for Ratatoskr to resurrect is not such a big deal, since it won't rock the main pantheon of characters so much. I also get a little fanboy-y about the idea of her and Sid being in a big brother-DRK/little sister-reborn ancient dragon relationship. Plus it's mentioned that she has a "great spirit" in her by the Vanu shaman, and I find it hard to believe simply being the child of a man who ingested dragon's blood would be enough to warrant "great spirit" designation. I may be reading too much into it though.