What is preventing this from happening?
I know Richard Page & Steve George, who are the main guys behind the band, have been friends since childhood and still work together from time to time. Pat Mastelotto is still active as a musician with King Crimson (or Crimson Projekt as they now are since Robert Fripp's retirement). I know Richard even recently discussed a Mr. Mister reunion in interviews.
They have their choice of guitarists in Steve Farris, Buzz Feiten and even Trevor Rabin played on "Pull..." (although I doubt he'd commit).
(If none of those 3, i'd love to see Steve Lukather take over the guitarist role in the band.)
Anyway, why hasn't this happened?
Would it ever?
Niv