The delay between Sonic 4: Episode 1 and Episode 2 has been extended, and it was something of a surprise that the second entry is coming at all. However, taking a page out of the episodic series of Half-Life 2, it doesn’t seem like a trilogy of episodes is confirmed.
“We are looking forward to hearing the feedback from the users for Episode 2, but we are currently not planning to release another episode,” said Sonic Team producer Takashi Iizuka. “We just want to see how the users accept this episode.”
Source: Digital Spy