Force immortality isn't a new concept, we saw it even in the very first movie. Big difference being that Sith cannot travel freely across the galaxy like dead Jedi can, they need to take control of a body or trick someone into entering, where ever their spirit lays, (normally their tomb or temple). It is also possible for them to return from the dead like Marka Ragnos and Exar Kun both did in the time of the New Republic (Luke's Jedi Order), but they again are bound to being spirits.
Palpatines method was way out there though, he just possed clones of himself so he could continue on forever, even though all his clones died he still attempted to posses Anakin Solo, but was stopped by the dying cyborg jedi Empatojayos Brand, who took Palpatine with him into the force when he finally passed away. It was a very long and to be honest, dull story. It doesn't quite come close to the Thrawn Trilogy, but it became apprent quickly that post-clone wars, writers love using clones to bring back dead characters. Slightly annoying as a clone will never be the same person as who it was cloned from, even Boba isn't exactly the same as Jango, and neither were any of the clone troopers, but hey star wars writers love to take what canon they need for their stories then ignore the rest because it disproves their stories.
