Sega held a party for Sonic’s 25th anniversary at one of their Joypolis parks in Japan earlier today, which they live streamed online. There they showed new gameplay of Sonic Boom: Fire & Ice and announced that the next main Sonic game will be released in 2017.
But the coolest thing that was shown was the original concept art of various Sonic characters. While a bunch of it has been shown before (and can be found on our wiki), we also get to see never before seen design sketches of characters like Shadow the Hedgehog, E-102 Gamma and Big the Cat for the first time. You can see all of the concept art that was shown above.
[Source: The Video Game Art Archive]