Bigger, ultimate evil bads is sort of a template way back from the serials that Star Wars is based on. Good ole Flash Gordon's Ming the Merciless was an uber-bad guy, making Plageuis the uber-bad guy would work in the tone. The series does have an inherent mythological ultimate evil vs good at it's core. The bad guys are personified by the Sith and the Dark Side of the Force. I really don't want to see a non-Sith as the main bad guy for the entire series, they should just be straight-up evil Sith. No inner-conflict, no mercy. What Star Wars does is take ideas and concepts that are cheesy and present them as straight forward dramatic stakes, which would take the bad guy with resurrection powers and make it deathly real. As for grabbing Plagueius, it keeps the whole series as an overriding whole as opposed to a random bad guy who has no connection to Palpatine or Vader or the Sith.