Producer Andrew Kreisberg has said that they definitely want to meet the Mirror Master next year. That makes it fairly obvious that hell be at least be making a small appearance, and maybe be a recurring villain throughout the season, or quite possibly the overarching main bad-guy. Hes a villain who couples advanced technology with mirrors in order to use them for an array of purposes, such as hypnosis, trapping people inside them, transforming his appearance, and travelling between dimensions. The prospect of him turning up in season 2 has already got fans excited, and if done right he would very much fit into the show and could help serve as a catalyst for expanding the world beyond just one-dimension, while proving an excellent foe for Barry.