Ryuubee will most likely die, I think. Probably Saeko too, but W isn't the kind of show that would kill off characters for cheap shock value nor is it super dark and serious. If any of the heroes dies I think it would be Philip. He'd be easier to give a "Disney Death" like Tsukasa or Shinji
I'm still keeping my theory on how it'll end: Somewhere in the ending, Shoutarou's on his typewriter, Phillip's doing something...Phillip-esque and Akiko's taking care of Mikku.
I'm still not sure how to work Wakana, Saeko or Ryubee into it yet, though.