A couple minor things that kinda bugged me about the physics and movement: 1) In your damage test video, you show Sonic landing on his back and righting himself after taking a hit. I know this is kinda how it's done in modern titles, but since you're saying you want as close to 100% recreation of the physics as possible, this is going to irk those that aim for the standards. It's also kinda antithetical to the design Sonic Team had. they wanted you to realize you got hit (which is why they took control away until you landed on an object or ground), but to be able to immediately 'get back up' and try again, which is demonstrated by not having any kind of animation for laying on his back (look at what this kind of thing did to Sonic 2 in its early state. It got axed pretty quick). 2) Ring bounce physics. I know this is going to be hell to get right, considering the differences in 2D vs 3D, but again, this is going to be key. Getting the lost rings to spawn in the proper places and with the right initial velocities, and making them bounce correctly will go a long way to showing you care about the details. 3) Special Stage: While at first, the lack of group transformation bugged me, I figured it was cool since other things (like touching red spheres) were also unimplemented.What got to me more was the transition between backwards and forwards movement. In the original, there's a little pause, which you've kinda captured, but it seems to be more of a slow down, stop, start sequence when you take the time to observe it. It's hard to see with the way the Genesis implemented it, but it's there if you look. You might want to cut a quarter of the pause time at the front and back to put in this velocity curve to help it feel more natural.