Xayide wants to supplant the Childlike Empress through Bastian despite her status as a Fantastican? The beginning of the book defines all Fantasticans as respecting the Childlike Empress and never wishing to overthrow or harm her because she views them all as equally important, and never judges or commands them. Y'gramul the Many is a prime example of a foul creature who respects Auryn and the Empress because she respects him and his nature. Xayide goes against this previous characterization of native Fantasticans by wanting to supplant the Childlike Empress and rule through manipulating Bastian as a puppet emperor. She could be the same kind of being as the Gmork who have no world of their own and thus want to take over or destroy worlds like Fantastica, but there is no hint of it except that her actions vaguely imply it.
Xayide has no expectations of actually ruling through Bastion. She recognized the City of Old Emperors when she followed his trail there, meaning that she knew full well what would happen to him. She was just trying to drive him to his own destruction For the Evulz. (Also, Ygramul is a she.)
It's also worth noting that Xayide shows up literally after Bastian has been sulking after a minor argument with Atreyu and Falkor and has deep-down wished to be seen as dangerous, powerful and someone who can't be pushed around. In a way, she's really just fulfilling that wish for him, whether she realises it or not; every action he takes and that she manipulates him into thereafter works towards making him come across as dangerous, powerful and someone who shouldn't be messed with. Similarly, it's probably no coincidence that she's destroyed almost immediately after Bastian has escaped the City of Old Emperors, renounced his previous wish and has instead wished just to find someone who will treat him as an equal part of a community; she's no longer needed.
Xayide likely just wanted the power and saw Bastian as a way to get it. Even if Bastian were lost to him, she still could act as his proxy, claiming he wanted privacy (as the Childlike Empress rarely was seen). After all, her modus operandi was to get her way through Batman Gambits.
