The stories in this series have been getting far too shallow. There's no problem with it being "too deep" or "too dark"; the writers have just gone in the opposite direction of Adventure 2 and 2006 instead of writing deep stories that fix the Plot Holes and other logical inconsistencies that the "darker" game tend to have.
The series is resting too much on the laurels of its '90s games. How many times has a new Sonic game been held up as the "revitalization" of the series, a "return to its roots"? The Advance games, the Rush games, Sonic 4, Colors, Unleashed (daytime), Lost World, and especially Generations: I want Sega to actually publish a Genesis-style game or two and then move on. The Adventure and Heroes games were actually good! Modernize that!