I recently started working on a mod in which the player is able to converse with spirits trapped on this plane, and is then given tasks to set them free. I know that Dunmer are cremated, Bosmeri eat dead outsiders, and Imperials seem to prefer burial, but I don't know much of anything about the other races, or even how complete my information is. Do the Dunmer in Cyrodiil have their ashes shipped back to Morrowind, or simply allow themselves to be buried? The Nords, I understand, are returned to Skyrim?

I'm not really massively fussed about the details, I can easily make things up as I go, though I'd prefer that what I do make doesn't fly in the face of established lore.

They get buried in the local cemetery or have their remains shipped back to be buried back home in their hometown or kingdom. Depends on the practice of the family.