If you are thinking of adding another character how about Metal Sonic? He could actually bring some interesting gameplay elements to the table if you remember that just because he's a robot Sonic doesn't make him his clone. He has different acceleration, different jumping heights, different speed, and he has the black shield and you could possibly use that cannon for something like a laser. He too could probably go through Sonic or Knuckles paths. Also how do you plan on having Amy going into Sonic's Lava Reef Act 2 Path? How would it work if Knuckles isn't there with the bolder? And who do you plan on replacing Amy with? Sonic? What happened to that Sonic 3 and Knuckles redundancy edit, there has to be additional data that can be canned for the sake of having another character in there.