I bring this up because I am one of those people who believe Sonic has lost a lot of the magic that he had in the past. Now, I won't say that the Sonic games to come after Sonic Heroes are bad (yes, I consider Sonic Heroes to be the last great Sonic game), but I will say that they've lost focus. The blame for this mainly goes to SEGA and Sonic Team.
You see, I don't believe that developers should listen to their fans. Lately, Sonic Team has been so focused on trying to please fans by creating different types of Sonic game, but really they're just adding gimmicks to some solid formulas. It's as if Sonic Team (ST) really want to make a more simple, straight-forward Sonic game, but finds that its not enough. I find this to be a huge fault in their judgement, if it's the case, for straight-forward design was what Sonic was all about, even in the Dreamcast era. The player was given a goal and the abilities to achieve the goal, and then sent on their way. Adding gimmicks is just hurting the formula.
Now, I know what your thinking, if straight-forward game design is what I want, then why wasn't I pleased with Sonic Generations? Well, because SG didn't have good level design. You see, Sonic, like Mario, was a master of the fundamentals back on the Genesis. Level design was top notch, the difficulty was balanced, and it was always fun. SG sacrificed fun and good level design for fanservice; and that's no good.
To be honest, there isn't much more to say when it comes to criticizing modern Sonic games that hasn't already been said, so I'll just say what I would've done if I was running the show over at SEGA:
First, after Sonic Heroes, I would have made 3 main lines of Sonic games: Sonic Adventure, Sonic Advance, and Sonic Heroes.
The Sonic Adventure formula was perfected in Sonic Adventure 2, and I would have kept the series going with more numbered iterations that would continue the stories of Sonic and his friends. I honestly don't get the hate for the Sonic family of characters; some can be annoying, but they all have their charm; in fact, my two favorite Sonic characters aren't the main three, but Shadow and Rouge. I say keep the Sonic family growing; though there would be limits, heh. Still, the various gameplay styles brought about by the multiple playable characters was a major part of what made the Adventure games so great; that would be the main draw of the series. Also, console-specific control gimmicks could be added to certain characters if the big guys upstairs demanded it; though these would be small or added to new characters in order to keep Sonic running and Knuckles fighting.
The Sonic Advance series developed by Dimps was fantastic, and I would have kept it going, adding elements like more playable characters and more dynamic bosses. Sonic Rush was a great follow up to Advance, but its a shame these games didn't keep going. This series would be for those who desired the "classic" Sonic experience.
The Sonic Heroes games wouldn't have as many iterations as the others, as they would mainly be fanservice-y team-up games. Perhaps in the second Heroes game, Sonic, Shadow, and Silver would be a team, and Rouge, Amy, and Cream could make up another. As the series went on, fans would get excited just to see who was going to team up with who. The gameplay style would remain the same, but Heroes would have its own story canon outside of the Adventure series.
So, there you go, that's what I'd do with Sonic if I had the power. Honestly, the newer Sonic games haven't even been that bad, after all, they're still well made games. It's just that when it comes to gameplay, Sonic either is given gimmicky gameplay styles or runs so fast that its impossible to provide good level design.
I think SEGA can go two ways: either keep doing what they're doing, and constantly trying new things to get the fans back (after all, they're trying, and the games are never terrible), or reboot Sonic (again) with a new game a la Sonic Adventure or Sonic Advance. I know that they've already tried this with Sonic 4 and Sonic the Hedgehog (2006), but I think they should really pour their money into a big project. Sonic Unleashed was really close, but suffered from a poor overworld style, so-so level design, and the werehog (which wasn't terrible, but still felt unnecessary).
Also, SEGA has to get Sonic music back to Genesis days. It's not terrible in the modern Sonic games, but I want a return of legendary music in my Sonic games.