I'm glad too. Seeing the game's already doing well, they could have just move to next project but they chose to support by giving more free DLCs next years. Very nice of them.
I imagine the next Sonic game will be somewhat recycled from Frontiers.
And that is not a bad thing. Something I think they should of done for Sonic Forces. If Forces was recycled from Generations I'd say be a huge improvement.
But while the game was well received, I still hope they improve on the gameplay a bit. Not only would I love an open-world Sonic game with physics and momentum like Sonic Adventure, I found the new control scheme a bit confusing. I found the "Skills" just flashy and mostly pointless at times, especially in Cyber Space where skills were never used at all and the gameplay didn't feel as fast as it could be. Something that could of easily be remedied if they leaned the gameplay more towards how Sonic Speed Simulator did it.
At least, I hope Sonic Team are working on the next Sonic game. I am looking forward to Frontiers DLCs... Yet to get the Christmas Sonic DLC though, is that a newsletter subscription thing like the SA2 shoes or what?