Absentee Actor: Black Knight is one of six games in the franchise where Dr. Eggman is completely absent (and the only one featuring the main Sonic cast including Sonic himself), the others being Sonic Eraser, Sega Sonic Popcorn Shop and the three Tails spinoff games.
Franchise Killer: Black Knight's poor critical reception and commercial failure seems to have killed off this series of spinoffs after two stories.
What Could Have Been: Secret Rings began life as a Wii port of Sonic the Hedgehog (2006), with the developers experimenting on playing the game largely with motion control. Development then switched to making an all-new Sonic title exclusively for the Wii, designed to take full advantage of the hardware, and thus Secret Rings was born.
Two missing members of the Knights of the Round Table in Black Knight are actually playable in multiplayer, with Silver the Hedgehog as Sir Galahad, and Jet the Hawk as Lamorak. Presumably, this means the two were considered for actual story roles, but for some reason or another didn't make the cut.