Remember that Sonic Rift hoax? Well my dream Sonic game is kinda similar to that.
5 playable characters, everyone plays differently. The story mode uses SA1's style of progression, and an online mode where you can race up to 4 players. There is very little to no nostalgia pandering.
Sonic: Closely follows the boost formula, except you build up speed as you go, like in Sonic Advance 2. Level design is a mix between Unleashed and Generations.
Tails: Mech shooter stages, controls closer to E-102 Gamma in SA1 than he did in SA2.
Knuckles: Treasure hunting with a focus on combat. Kinda similar to the Werehog stages in Unleashed.
Shadow: Plays like he did in SA2 with level design closer to Sonic's levels in SA1.
The fifth character is the Avatar, except he/she has no part in the story. He/she is used in the online mode where you race other people in a similar fashion to Tails' stages in SA1. I would also like to see some more customization options for the avatar, more hair types, the ability to change your avatar's build, and more customizable clothing. I would also like if the female avatar was given a two-piece bodysuit to begin with rather than a one-piece to allow for more customization.
The story should be something along the lines of SA1 or Unleashed, as everyone acts in-character and things don't get too dark and edgy or too lighthearted and kid-friendly. All cutscenes are either pre-rendered or using game assets. No Sonic Forces or TSR text boxes here.
There is a hub world for every location that is visited, an in-between of Unleashed and SA1's hub worlds.
For levels, some themes are returning, and some are brand new. A few I would like to see are:
Beach setting (similar to Emerald Coast or Wave Ocean)
Nighttime city (There's a lot of potential for this one, I'd prefer a mix between Studioplolis and Speed Highway)
Ruined/abandoned area (The Land of Darkness from the OVA comes to mind)
As for side characters, it wouldn't include characters that would seem out of place or are just there for no reason, a good example of this is the inclusion of Silver or Blaze in certain games, they live in their own dimension/timeline and not in the main one. In Forces and TSR it feels like they were just slapped in there for fan service. With that being said i wouldn't mind a spin-off game for Silver.