The 1997 N64 racer Diddy Kong Racing almost had a couple of sequels. There's the well known Donkey Kong Racing, a title Rare announced at E3 2001, which failed to see the light of day once Microsoft acquired the series developer. But there was another one too. This second stab at a gorilla-racing sequel was called Diddy Kong Racing Adventure and it was being developed for the Gamecube not by Rare, but by Climax Studios (Silent Hill: Shattered Memories, Sudeki).
YouTuber Andrew "PtoPOnline" Borman revealed the following footage of this canned sequel in the video below:
In development around April 2004, Diddy Kong Racing Adventure was set after the titular Kong child defeated Wizpig, who has returned with a Kremling army and turned Kongo Island into a colossal racing circuit.
The game was to consist of 16 villages, each with three courses. Players would be able to freely roam a hub connecting the various villages, and in a De Blob-like twist, each village would begin drained of colour, but through completing races you'd restore the landscape to its former festive glory.
One unique mechanic Diddy Kong Racing Adventure conceived was being able to change the position of how your character rides their vehicle. Should you need to make a sharp turn, you could hang onto the edges of your vessel so it can careen on two wheels, for example. Climax also proposed a parachute to glide across gaps - mind that this was seven years before Mario Kart 7 made that a mainstay for Nintendo kart racers.
Unfortunately the game was never completed. In fact, it's likely that Nintendo never even commissioned it and Climax simply whipped up the above prototype just to pitch it to the Kyoto-based company, who then turned it down.
As such, the series never really lasted long enough to be a series, though there was a 2007 DS remake of Diddy Kong Racing. Apparently it didn't hold up so well a decade after the original game's N64 release.