The only thing that really bothers me still in the Sonic fanbase from what I see, is people saying that Sonic needs to "go back to it's roots and not re-invent itself all of the time", then they post their idea for a new Sonic game, and it's literally a re-invention/reboot.
I think Sonic just needs to balance the two. The best period of Sonic's career after the Classic games, objectively, was the "Unleashed Trilogy". Unleashed, Colours, and Generations, had gameplay mechanics that were consistent, but the three games also did their own things. Sonic had a clear identity during this time, and the reception to these games showed that. People knew what Sonic was about - fast and fun platforming.
Since Generations though, they have been experimenting a bit too much. Sonic Boom is it's own thing so I don't mind that, they were explicit about how it's a new branch of the franchise that will co-exist with the main series, and will be different from the main series, and I don't mind that. Granted, Rise of Lyric was still a terrible game regardless of that, and Shattered Crystal was "just OK". But I don't mind Sonic Boom being drastically different.
The problem comes in when Lost World was also experimental. And I don't mean in how it changed the gameplay from Generations (which was gonna get stale eventually - especially for people with the PC version, with it's mods). What I mean is, Lost World had a really cool idea for a new gameplay style, and there are points in the game where it's really well implemented. I honestly think Lost World's controls are fine and only need a bit of fine tuning. However, the game flung you into forced and clunky 2D sections, snowball mini-games, intrusive pinball mini-games, and all of that, giving little room for it's core gameplay to shine. And it isn't like a Classic Spyro game where at least the mini-games are fun and polished and often optional, they are mediocre and boring and mandatory.
I like Lost World, but it had a lot of problems, and it's main issue is that it sort of took Sonic Team's "over-experimentalness" that they have demonstrated across the 2004-2007 period, and represented it in a single game.
IMO, the next Sonic game needs to pay homage to the past of the franchise and bring back certain elements that people liked about past games. But it also needs to be ambitious. Generations is one of my favourite games, but it wasn't ambitious at all. It completely pandered to nostalgia, and that was fine because it's what the series needed at the time, and the game was cut and dry about it. But the next game needs some of the general consistency and simplicity and old-school feel of the Classics/Generations, mixed with the modern ambition of games like Unleashed. And I have confidence that it will achieve that, as it has been in development for a while.