I'd prefer a Sonic 5. Classic 2D action with enhancements here or there but with the main focus on level design.
Just keep it to Sonic, Tails, Knuckles and Dr. Robotnik too (don't remember when or why they changed it to Eggman I hated the change).
An anniversary should celebrate what they did right, and I didn't personally play any decent Sonic games after Sonic & Knuckles, they were all awful, barring 4, which was okay,