Sonic Riders isn't the most well-designed racing game out there, but it's a fascinatingly technical attempt at breaking from your average kart racer gameplay. It's no surprise that the game has developed a cult following, and even a steady competitive scene, spawning several different overhaul mods that tune the mechanics and content in different ways from one another. Riders is fun, stylish, and scratches itches that haven't quite been reached by the Sonic racers that followed.
Unfortunately, not only is Sonic Riders unavailable through legal means nowadays, but the PC port simply isn't great. Several graphical effects are absent or broken, multiplayer is limited to 2 players instead of 4, and numerous unofficial patches are required to get it up to speed on modern systems.
While Riders itself has game design issues, such as the lack of a tutorial, and some mechanics being more obtuse than they should be, fixing those is likely outside of the scope for your average Sonic re-release. It would be more than enough to see this game receive engine-related fixes and tune-ups to make it cooperate better with the computers of today - especially, say, Steam Decks - and to play it in a form much closer to its original GameCube release.