There was no real Sonic game for the SEGA Saturn, so it wasn't until the Dreamcast that a real 3-D Sonic title was made. There was work on a game called Sonic X-treme, but that got canceled and shuffled between systems. This is unfortunate, because Sonic's love interest in the game was planned to be a new character called Tiara Boobowski, which is ridiculous.
In making Sonic Adventure, the team redesigned the characters, making Sonic lankier and more cartoony. They also added a lot more characters, and decided that giving each one their own level types would be more interesting than each playing as similarly as they had in the past. They also took trips to South America for research on temples and jungles in order to plan the appearance of areas in the game like Mystic Ruin.