In the original Sonic CD, there was an extra feature called D.A. Garden where players could visit an extra sound test with a world that could be changed and dragged around. There were also little sprites like Flickies, Metal Sonic, Tails' Tornado, and Robotnik that would fly around the screen at times.
This feature was moderately brought back in Mania where the sound test was still there, but it only featured a spinning world with no interactibility.
CD.A. Garden is a mod I've recently decided to make. While there is still not interactibility, this mod brings back the spinning D.A. Garden world from Sonic CD, a new HUD for the stage that looks like the original, Metal Sonic/Tails/Robotnik that fly around the stage, and a bright world (instead of the sunset). I worked with the limitations that I had.
Do note that you can access D.A. Garden by getting 24 medals from Blue Spheres OR by using the dev menu.
Please contact me for permission if you would like to use this or any of my other mods in a pack.