Oh dude, Sonic CD is a fucking
crucial game. My all time favorite Sonic game. It's not as long as Sonic 3 with Sonic & Knuckles but it has amazing graphics, gameplay elements, and music. It's got 4 different endings and you have to like, go into the past and destroy robotniks machinery in the past, and if you do that in the first two zones of each level, when you warp to the future in the final zone, it's a " good " future, meaning technology is still there, but in a good way and the environment is propering, but if you don't do that, robotnik takes over everything and it's a dark polluted " blade runner esque " future.
that's one of the " dark " future outcomes