Sonic has had a rather mixed year in 2017. The iconic hedgehog received a nostalgic filled throwback with Sonic Mania but it was followed by a critically panned Sonic Forces. 2018 seems to be rather important for the franchise as Sega is hyping up an announcement for it at SXSW panel in March.
Sumo Digital have previously worked on the Sonic All Star Racing Transformed game and in an internal presentation, that was leaked online, they talked about working on a Racing game that was still unannounced. At that time, people thought that they were working on Sonic Kart Racing since they have a history with the franchise, however SEGA PR denied that any such game is in development at that time.
The most recent rumor is from British Toy Maker Zappies who are allegedly working on creating toys for the upcoming Sonic Racing game. They have previously worked on toys for Sonic All Star Racing Transformed so they do have a history with the publisher, however it remains to be seen if this is a simple rumor or there is some truth to it.
Sonic fans will have to wait until this March in order to hopefully get an announcement about what to expect from the series going forward.