A picture of a sign at the Jazware (toy manufacturer) booth at International Toy Fair Nürnberg 2014 is making the rounds as it confirms a new Sonic title in 2015. Furthermore it lists it as coming on PS4 and Xbox One in addition to Wii U (via Sonic Stadium).
Sega signed a deal with Nintendo to keep Sonic exclusive to Nintendo platforms for three games (the first two being Sonic Lost World and Mario & Sonic at the Sochi Winter Olympics), the third game is still unannounced. It's unclear whether this "Sonic 2015" title is that third game or if there is one more game coming in 2014 on Nintendo platforms. The sign appears to rule that out.