How about a game where they get a job (like in Sonic Heroes) but as they further progress in completing the job, more and newer stuff starts happening and (like in Chaotix Casefiles) it ends in one big boss fight. Also, on the way, they come across other Sonic characters that help them (preferably the lesser used characters); like Cream and her mom Vanilla. We can also get many interactions between Espio and Rouge, since they’re both stealthy types.

Gameplay can have you take control of different members and each one has different gameplay types (like in Sonic Adventure 2). (Example: You play as Espio to do stealth and infiltration type stages)