Sonic Lost World had the Parkour system with a mostly otherwise-Adventure moveset, with the spindash effectively acting as a boost. It did not please everyone. This kind of thinking is what SEGA do already, and it's exactly the problem - you are not going to please everyone with one game. Different sections of the fanbase want entirely different things. Hell, there's no reason they can't just use a different character as the mascot for each of the different sections. Keep Sonic for the current, Boost-style games; have Shadow be the face of Adventure-style games; and let Mighty have Classic-style games. If there are three people sitting down to eat, and one person is a carnivore and wants a steak, one person is a vegan and wants a salad, and one isn't hungry and wants a bottle of rum, you're not going to please them all by offering a steak soaked in rum with some salad sitting on top. Your best bet is to serve them each what they want individually.