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.
Unrelated to Adventures, but thanks to this post I rediscovered the Sonic Generations Unleashed Project mod (alongside some cool looking add-on mods). I saw a single video on the base mod years ago and completely forgot about it until today, so thank you for this, I guess, and also thank you to the person who posted the link to the mod site.