In an interview with Game Informer during E3, Sonic Team president Takashi Iizuka offered clues about what the future holds for the blue hedgehog. He said:
2017 was a big year for Sonic. The next big year for Sonic is 2021. That will be Sonic's 30th anniversary. We are preparing for it now.
The series' last major year was 2017, which saw the release of not only Sonic Mania but also Sonic Forces, celebrating the series' 25th anniversary. Iizuka already confirmed at SXSW this year that the next mainline Sonic game developed by Sonic Team is already in production.
The interview also asked about Drop Dash, one of the moves added to the hedgehog in Sonic Mania, particularly its addition to the Switch re-release of Sonic the Hedgehog through SEGA AGES.

Drop Dash was added to Sonic 1 because of its popularity in Sonic Mania
“Drop Dash in SEGA AGES was my request,” he said. “Drop Dash is well received now because of Sonic Mania. That's why I asked for it.”
Iizuka also indicates that Drop Dash will remain an element in future Sonic games and re-releases of the hedgehog's older 2D games.
“Yes, I think so. We had a good reception with Sonic Mania, and that's why we implemented [Drop Dash] in Classic Sonic in Sonic Forces.”
Until 2021 arrives, fans' main focus will be the hedgehog's first film for the big screen, Sonic the Movie.