Just like Capcom with Mega Man 9, Sega had a plan last generation to make a 2D Sonic sequel in the vein of the Genesis originals. The result was Sonic the Hedgehog 4, and though Sega's intentions were good, the execution left a lot to be desired. One of the questionable decisions was to make the game episodic, with the potential to expand the game for as long as there was demand. It only got up to the second episode before the project went completely dead.
At one point, someone discovered unused cutscene data in Episode II with the text "To be continued," suggesting that an Episode III was in the cards at one point. A fan recently contacted former Sega digital brand manager Ken Balough on Facebook to ask about this information, to which Balough replied:
The unused text was because there was going to be an episode 3.
For reasons I can't discuss, Episode 3 never came to pass. But the idea back during Ep 1 was to make a full trilogy.
He further clarified:
Yeah. The project will likely never see the light of day.
For many who played the two Sonic 4 chapters, this isn't terrible news. You need only to go back and re-read some of the reviews for them to realize that Sonic 4 didn't come close to recapturing the essence that made the Genesis games so popular.
Then again, Sega would do well to attempt a proper return to classic form, but that would require a different team that has a better understanding of old-school game design. There are people in the fan game community who could build a pretty solid 2D Sonic — Sega even hired one such fan game designer to remaster Sonic 1, Sonic 2, and Sonic CD! The talent is there, Sega! All you have to do is ask!