First revealed on July 22, 2016 under the codename "Project Sonic 2017," Sonic Forces was officially unveiled at SXSW in March 2017.
Sonic Forces takes place in a world on the brink of destruction at the hands of Dr. Eggman. Sonic and his friends fight a losing battle to save the world until Classic Sonic mysteriously resurfaces once again from Sonic's past to aid in the rebellion.
The gameplay in Sonic Forces is broken up in to three styles. The first two are very similar to Sonic Generations; Modern Sonic is controlled from a behind-the-back viewpoint, where players use boost to smash through obstacles and run at high speed. Classic Sonic is more similar to the original Sega Genesis games, with a side-scrolling viewpoint and a greater focus on jumping around on platforms.
At the time of this writing, nothing is known about the third gameplay style in Sonic Forces. A third character has been teased in silhouette, and it's likely they are related to the third gameplay style.