We got a sneak peak of the third character in the upcoming Sonic Forces game during Nintendo Direct and it looks like to be... some wolf guy?

Sonic Forces 3rd character teaser

It's obviously a tease. There's also rumors floating around that this is really just stand in for a custom character creation tool but I'm not buying it.

There's also a little bit of plot points and new screenshots according to an interview with Iizuka-san in Weekly Famitsu magazine

The story portion basically reveals that Eggman has conquered 99% of the world and has used a special new power in order to do so.

Sonic Forces in Weekly Famitsu Magazine