Sorry, I refuse to accept that the Waitress was also... *spoiler*
...Terpsichore.
My interpretation based upon Zeus's declaration that "other people have dreams". Also, Terpsichore (Kira) only asks Zeus for "one night" to be together with Sonny. Zeus relents enough to grant her "one moment" which can last "forever". He means a symbolic "forever" not a literal eternity, which is to say, an indelible memory -- thus the phantasmagorical roller disco finale.
The "Nine Sisters" on the mural from whom the Nine Muses adopt their mortal forms were not the Muses' true appearances, rather those were the likenesses of nine normal human models posing as muses. The waitress who shows up at the end is not Terpsichore the Muse of Dance, rather she is the "mortal" Kira (a model from Australia) whose identity Terpsichore had mimicked. As a Greek goddess Kira's true form is too awesome and resplendent to be perceived safely by human eyeballs (as anyone familiar with the myths of Semele or Psyche knows).
After Sonny told Zeus that he loves "Kira", the All-Knowing Olympian then influenced (or had Terpsichore inspire) the mortal Kira to find a job at XANADU. Keep in mind that Muses will adopt whatever persona is necessary to spark artistic inspiration in the particular mortal they have targeted. Terpsichore fell in love with Sonny because that is what would have happened to the mortal Kira had she spent that much time with Sonny.