I think the problem is thinking that speed is the main thing.
In genesis games, most levels were built for platforming: enemies to jump on, gimmicks, springs, collectables, bosses... They just happen to have some high speed sections to take advantage from Sonic speed and physics. But they werent the real focus.
Adventure 1 almost nailed the 3D formula. Very great mix of action/platforming, speed and plot. Sonic Adventure 2 started the focus on speed with Sonic, with less inspired levels (except Final Rush) and the plot was just OK.
Sonic Heroes (PC) as its own became my favourite 3D Sonic game. Levels were long and awesome, also the music. The gameplay was really good too (the fact that you play with all 3 friends at the same time dissolved the focus on speed). The plot, just a bit worst than Adventure 2. Overall, is a really great game.
Then Sonic 06 happened, and because they failed, SEGA though the should abandon Adventure formula and go for a new one instead of improving it. That was the mistake.
Then we got just great games, but lacking the true potential of the franchise.
I think Sonic Boom, despite the character designs is getting back some things that made great the past games.
So, less speed, more platforming. It's just that. I dont care if its sandbox or not. Just more platforming and higher difficulty that actually needs skill from the player.