Well this came out of nowhere. Animation writer Dave Polsky (South Park, My Little Pony: Friendship is Magic) mentioned in a recent convention panel that he is writing a new Sonic cartoon called "Sonic Boom":
Moments ago at his Trotcon Q&A Panel, Polsky revealed more information about “Sonic Boom” – we’re still waiting to see if our source can elaborate more, but early reports indicate that it is a “Pony-esque reboot of the cartoon series”. What, exactly, that means is currently a mystery – Sonic has had no shortage of cartoons over the years, and each one of them involves completely different settings and supporting characters. Any one of them could be rebooted, and it’s worth mentioning that one of Polsky’s contributions to MLP was, apparently, writing music for an episode, so not even Sonic Underground is out of the question at this time.
Right now, you know about as much as we do. We’ll have more on this as it develops.
UPDATE: Our source has offered more information now that the panel is over regarding “Sonic Boom”. All Polsky would say is that it involves Sonic, Tails, Knuckles and Amy Rose, and would lean towards “comedic in style”, making it sound unconnected to any previous Sonic cartoon.