When it comes to describing a ruminative piano ballad, there are a few synonyms that come to mind. One could call it a contemplative or reflective piano ballad, as it often evokes a sense of introspective thought and emotional depth. It could also be described as a pensive or melancholic piano ballad, as it often conveys a sense of sadness or introspection. A somber or wistful piano ballad might also be an appropriate description, as it often leans towards the mournful or nostalgic. In any case, a ruminative piano ballad is a beautiful and introspective piece of music that lends itself to deep thought and reflection.