Sega's biggest mistake was not selling Sonic to Nintendo when they asked for it. Sega wanted to sell the whole house but Sonic was the only mascot worth saving SMMFH. Nowadays Sonic is a whore, multiple studios are making Sonic games, each worse than the other. They need to find one consistent game developer (hopefully one affiliated with Nintendo) and just stay with that. Stop licensing the brand to every Tom, Dick and Jane and take the time to craft great games. They should focus on establishing each of the Sonic characters as separate franchises, as Nintendo does with their mascots. Let Sonic be the traditional adventure game, Knuckles can have more of that Crash Bandicoot vibe (speaking of which when was the last time Crash got a game ?), let Tails or Robotnik be the strategy game and go from there.
The way Nintendo controls Mario, Luigi, Yoshi, Wario, etc. that's how Sega Sammy has to do it. They could revive Sonic in a major way if they dedicated that level of focus and creativity to the franchise. The way Mario has Paper Mario you could do "2D Sonic" and really stretch that concept out. The possibilities are endless.
