I'm a bit late to post this, but this article weaves together many different sources (old magazines, internal Sega communication, Usenet reactions, etc.) to explain just how big Adventure was to Sonic and Sega fans in 1999.
I remembered this older article just recently - it outlines Nintendo's attempts to market the GameCube in America, with Kyle Mercury (who worked with Nintendo during the console's lifespan) shining some light on some of their inner workings at the time. Among other tidbits, he claims that he recommended that he recommended Nintendo focus on Resident Evil 4 during a meeting with then-new hire Reggie Fils-Aime and that Nintendo began to feel spite for their own audience near the end of the era ("they felt as if they were being betrayed by the gamers they created," additionally figuring that they didn't need to appeal to their hardcore fans because they'd buy their games no matter what).
This is admittedly a bit old (originally posted on July 4th, 2020), but I still think it's an interesting historical piece about the game's unusual past (including the version that did see release: Special Champion Edition).
Sister Sonic, which was not meant to be a Mean Bean Machine-esque reskin of Popful Mail solely for overseas audiences but an outright remake for the Sega CD that would've released worldwide as a Sonic game (which turned into a more faithful remake after Popful Mail fans "vehemently protest[ed]")
A sequel to Sonic Chronicles, an RPG on the DS by BioWare that ended on a cliffhanger and inadvertently provoked a lawsuit from Archie Sonic writer Ken Penders that killed any further continuation
Sonic Day, a mobile game by Nitrome based on their hit platformer Jump Day -- Sega of Japan ultimately passed on it
(I actually wrote up a full summary of the video, but Lemmy wouldn't let me post it here.)