rpvee said:

It's also not like Sith/Dark Siders haven't had two names before in Star Wars history. He could still be Plagueis.

I find it hard to believe that the guy who thought about putting Jar Jar's skull into the new film as an easter-egg would make Plagueis one of the new main characters?
Also from what I've gathered Plagueis' full name is actually Hego Damask. Technically that name never appeared in any of the PT films, but Plagueis appeared in EU content before he was mentioned in ROTS so it's safe to bet that those novels were part of Lucas' marketing of the PT films. Similarly to how Grievious appeared in both novels, comics, and the cartoon show before he actually showed up in the film.