A dragon's aether is stored in their eyes, and Thordan and his Knights Twelve devoured Ratatoskr's eyes to gain her power, so her soul might not have even made it intact to the lifestream. Which is a lot more disturbing written out than it was in my head. Bahamut and Nidhogg could be out there, though. Some of Nidhogg's power is still bonded to Estinien, but we did explicitly see his spirit move on after the Final Steps of Faith, so who knows.

Of course, all of this only applies if dragon souls follow the same rules as everyone else's, which we just don't know.