Personally I'm kind of against it being Odium from a meta point of view- It would be a pretty big letdown to not find out what the force is in the Lost Metal book, and if it was Odium that would spoil that he gets out of Braize, which would likely be a major point in a upcoming Stormlight book.
Then again maybe I'm over thinking it and we won't even get to know what the force is until the next trilogy, if ever.
Actually, didn't he say that he had to wait for Stormlight 5 to be done before the next Mistborn trilogy? *puts on aluminum foil hat*