Here are some different thoughts I haven't seen mentioned anywhere in this thread. 1. OneOne-FourDee, Plagueis' trusted droid, was the only witness to most of Plagueis' experiments. FourDee could have been used by Plagueis in some way, to contain his essence, perhaps he was carrying some kind of creature that Plagueis had discovered in his experiments, that would make for an appropriate temporary host for his essence? FourDee was present for the entire "death scene" between Plagueis and Sidious, this would also explain why he seemed to just give up and not fight back, the moment he stood up briefly before falling back on to the couch was the moment his essence was transferred into whatever containment system he had placed inside of FourDee. 2. Plagueis as Damask was the one who had all the previous dealings with the Kaminoans, he was the one that suggested to Sifo-Dyas that they could create a clone army for the Jedi. Now while I agree that a clone of Plagueis would be lame, what if what the twist was, is that Sidious killed a clone! The real Plagueis never allowed himself to be alone with anyone, it turns out, not even with Sidious. Sidious killed a clone and Plagueis was allowed to slip into the background. Remember that the details of Hego's funeral were kept a secret by the Muuns. 3. Early in the book, Plagueis displays his ability to slow his hearts and breathing down to the point that his vital signs can not be detected. This takes place aboard the freighter he boards shortly after killing Tenebrous.