I love this game. I played the Dreamcast version as a kid so many times and recently replaced the PC version.
I learned that you can unlock metal sonic after 100% completing the game, but he is just a cosmetic for sonic and thats it.
My problem with this is that once the game is 100% complete, do I want to still play just to see a different cosmetic?
Maybe if there was some variety to make the levels a bit different or if he was able to be used after getting all the B rank emblems so there is still some challenge to complete.
Super Princess Peach does exactly the same thing.
The best reward for being good at a game should always be more game to play.