Because of how popular Sonic is, and because of how high the demand is for him in this game, it would be a huge mistake for him to not be included, and an even bigger mistake for him to not be announced before the game is released. Undoubtedly, there will be people who buy the game because they are fans of sonic, not of smash bros. Imagine how much press this game will get when Sonic is announced.
Also: Ice Climbers are back, which is cool, because there were a lot of rumors that they and G&W were removed.