Sonic Adventure was developed by Sonic Team and released in 1998 in Japan by Sega for the Dreamcast and is the first game in the Sonic Adventure series. One of its development titles was Sonic RPG, (although the final game was an adventure game, not a standard RPG).
Originally, Sonic was supposed to be able to engage his Super Sonic form at any time during gameplay, once he had gained all seven chaos emeralds during story mode. The proof of this comes in the form of an unused voice clip in the game, in which Tikal teaches the player how to become Super Sonic. The character reveals that Sonic must gain 50 rings before he can transform and warns the player to monitor their ring consumption closely, otherwise Sonic will lose a life when he runs out.
By hacking the 2004 US PC version version of Sonic Adventure DX, it is possible to access a number of unused assets from the Tokyo Game Show 2003 build of the game. One of the most notable of these is an early version of the character selection screen. The omitted menu section is fully functional if modded correctly and is labelled with the text, ‘Choose your buddy!’. Interestingly, this title is also the name of the background music composed by Jun Senoue for the finished character screen on Sonic Adventure’s official soundtrack, ‘Sonic Adventure Digi-LOG Conversation Original Sound Track’.
A two-headed mecha dragon boss was removed from the game before the final version, but it can still be found in the prototype / beta of Sonic Adventure DX that was leaked online. It was a part of the Sky Chase level, during Sonic and Tails’ approach to the Egg Carrier in Tails’ story section. It pursues the player up until the Egg Carrier fires its laser beam at the Tornado. You can find more info on Sonic Cult!
In addition, an unused animation for Tails has been unearthed in Sonic Adventure DX via modding. By replacing Knuckles with Tails in his story mode, players can activate a glitch in Casinopolis to trigger said animation. It is noticeably very similar to Tails’ swimming animation from Sonic The Hedgehog 3; suggesting there could have at one stage been bodies of water to swim in during one of his stages, or possibly even an entirely new water-themed level, a la Hydrocity Zone.
Thanks to FullMetalMC & BowserEnemy for some of these images!
SEGA Confidential images from SEGAKatana.com