>>529221006
>>529221154
Here: https://en.wikipedia.org/wiki/Unreleased_Sonic_the_Hedgehog_games#Sonic_Adventure_3
Sonic Unleashed was based on Dreamcast preproduction design docs of Sonic Adventure 3, which Naka wanted but failed due to Sega going third-party and Iizuka wanting to go in a different direction. They fully intended to continue with Adventure 3, not to mention Advance was also in development.
Source: pic