Topic: Hirokazu Yasuhara's Sonic X-treme Concept Art
I think you should make the gameplay based upon the concept art for most of the levels. While Hirokazu Yasuhara's Concept art wasn't really used in the game itself I think it would've filled in the gaps I mean lets be honest Andrew the game would be short if there's not anything that'll make the game any longer... I'm not specifically talking about the maps and how there arranged. I'm just talking about the level designs like one of the concept art for Crystal Frost levels for example: Crystal Frost's first level from the Sonic X-treme video is used as usual along with the other 3 acts from the concept art making it 4 acts then the usual 3 acts.
Anyways this was just an idea that came in mind. Remember Andrew take this as an opportunity to use some creative ideas in the game.