Since Qrow is called Qrow because his semblance brings bad luck and crows are seen as bad omens, maybe Raven's semblance is the ability to give prophecy or see the future because ravens are a symbol of prophecy. It is in keeping with the theme that their names are based on birds that represent their semblances because they are twins. It also explains how Raven knew that Beacon would fall, how she knew Yang would need to be saved, and how she seems so generally well informed. Maybe this even explains why she just up and left Taiyang and Yang. She could have had a vision or prophecy of something terrible happening to her family so she left to protect them. She may have even foreseen her tribe coming after her to take her back, so to protect her family she left them and rejoined the tribe without a fight. Other thoughts about her semblance, portals, or shapeshifting?