If it's possible for Darth Sidious to resurrect himself; then why should it not be possible for Plagueis to do the same? Could Plagueis be the villain of the new trilogy? Keep in mind, the expanded universe is nothing more than fan fiction. All that can be confirmed about Plagueis is that he was a Sith and he knew secrets about eternal life and creating life. The species that he was cast as in the books may not even apply to the films.