This is my concept for a Sonic the Hedgehog RPG, called:

Sonic Forces RPG: The Phantom Dreamer

You create an Avatar and adventure in the world of Sonic which is under a global assault by the Dream Stealers (outer dimensional aliens). Using your Phantom Ruby, you copy various characters from throughout the Sonic franchise to form a party and fight back the invasion as you explore the World, its History, and neighboring Dimensions.

Gameplay

You can freely switch who you are playing as on the map. Just open the Menu and select the Party Leader from among your current Members.

Combat

RPG Style with a Main Team of 8 Characters: you the Player Character and 7 Phantoms which are copies of other Characters.
- The reason for this number is so you can have a Party with each Member holding a Chaos Emerald, and your Character holding a Master Emerald.

When you get into a fight you pull the enemy into your Phantom Zone, which is the Lore explanation for the "screen transition" into combat.

You will unlock the ability to customize the Phantom Zone, allowing you to pick a Stage for the background and the music that plays when the fight starts for Generic Fights. You will be able to set this up per a Character. For example, if you are running around on the field as Sonic and get into a fight, the Phantom Zone could look like Green Hill Zone while playing Escape from the City. Alternatively, if you are running around on the field as Super Shadow and get into a fight, the Phantom Zone could look like Pumpkin Hill as E.G.G.M.A.N. plays in the background.

There will be various ways in game to unlock new Zones and Music to customize with, along with future DLC for content from future Sonic games.

There will be times when you team up with the Real versions of Characters. In those segments, they will take up a party slot, can't be customized, and will act on their own on their turn.
In general, after the first time you team up with someone you can then start using their Phantom, which is a fully customizable Party Member you can control.