This just in from the SXSW Sonic panel: Sonic Mania has been delayed from Spring 2017 to Summer 2017! Still no exact date revealed. According to the panel, the devs need more time to make sure the game is as good as it can be.
They said SEGA doesn’t want to disappoint its fans, and is making sure it is taking the time needed to make a high quality game. “Quality is our number one priority.”