Sonic X-treme concept art
From Sonic Retro
Over the course of Sonic X-treme's development cycle, a slew of concept art and other various images were created as the team behind the game brainstormed how best to go about producing Sonic's first appearance on the Sega Saturn. Some of these ideas never went beyond the drawing board, while others evolved into various forms of playable content until X-treme's cancellation.
General Concept Art
Sonic the Hedgehog
During the early development of X-treme, part of the team's ambitious ideas was to feature numerous playable characters, each with their own individual play style. Chris Senn, the game's lead designer, wanted Sonic to be controlled in an isometric view's similar to what would eventually be seen in Sonic 3D: Flickies' Island, but was convinced otherwise by lead programmer Ofer Alon.
Miles "Tails" Prower
Though not demonstrated in the below concept art, Miles "Tails" Prower's game play was envisioned to be that found in most modern 3D games, with a third-person, camera-behind-you perspective. When the game was focused onto just Sonic, the titular character inherited this format of perspective.
Knuckles the Echidna
To keep things fresh, the player was to control Knuckles the Echidna from a top-down view, the camera showing off the maze surrounding their character.
Tiara Boobowski, commonly mistaken for a hedgehog but actually a Minx Kitten, was meant to be the new character introduced in Sonic X-treme. First conceived as a princess, she later become the daughter of a professor, but each time was meant to be a love interest for Sonic. Though later game outlines made her nothing but plot fodder, she was originally meant to be playable, to be controlled in traditional side-scrolling platformer levels.
Included within the "PackageX" leak of 2009, a set of sprites for two characters, "Chaz" and "Gabby," were found within. They were created by Chris Senn for a personal, unrelated side-project using the X-Treme level editor, and were used as test art.
Hirokazu Yasuhara Concept Art
Near the tail-end of development for Sonic X-treme on the Saturn, Sega of America called upon veteran Sonic game designer Hirokazu Yasuhara to lend his expertise in the ever-problematic X-treme development cycle. His concepts continued to be used during the "Project Condor" phase of the project, but none were successfully utilized in a complete game.