It would not be anticipated by most. And what if he became so powerful, Palpatine did not really kill him -only his physical form. Much like Obi-Wan and Qui-Gon, Plagueis has mastered the art of becoming one with the Force after death. And what if Plagueis wants to completely destroy the Rule of Two and completely reform the Sith? Seems rather likely as he was killed by his apprentice because of the Rule of Two.

I think that somehow this character that is rumored to be an aristocrat obsessed with Sith artifacts (Grave Robber that holds Vader's mask in concept art) eventually learns the legend of Darth Plagueis. His journey leads him to an ancient Sith planet and Plagueis manipulates this character into somehow bringing him back to life and the physical realm (maybe he will possess his body, eh, that's kind of far fetched). Once this is achieved Plagueis will want to destroy any remnant of the Sith that remains, and even the Empire because it is the institution of his apprentice. Basically he wants to destroy everything that Palpatine helped create.

At the same time however he wants to attempt to turn Luke and his children (if he has any) and Han and Leia's children to the Dark Side because they are related to Anakin who defeated Palpatine. Or maybe Plagueis has this crazy warped idea of merging both sides of the Force, and one character will side with him to prevent the death of someone they love...again.

Some evidence to speculate about:

Lucas in a 1980 interview about the possibility of a third trilogy: "It deals with the character that survives Star Wars III and his adventures." It's a long stretch though...

Palpatine says this to Anakin in EpIII "To cheat death is the power only ONE has achieved but, if we work together I know we can discover the secret."

And in Return of the Jedi in Yoda's hut, before the 800 year old master passes, Luke says, "But, Master Yoda you can't die." Yoda responds to this by saying, "" Strong am I in the force but, NOT that strong."

What do you guys think? Imagine this. It's almost a Cold War scenario between what's left of the Empire and the New Republic/Rebels. Neither want to fight each other, but they are more than prepared to do so. Then we hear reports of someone (or something) killing Imperial Officers/Stormtroopers on a remote planet. Empire is quick to point fingers and tensions rise. Soon we learn that it is Plagueis back form the dead, and he wants two things. 1. To destroy everything relating to the Sith and the Empire and 2. He wants to talk to Luke Skywalker. What do you think about my crazy theory?

Hmm... maybe. It is the only other sith lord normal people would know about, since he's mentioned in Ep. III.