I translated Sega Of Japan's Sonic Advance 2 page and found a few tidbits.

Here are 2 pics that aren't on Sonic Team's page. Notice the Amy icon that shows up again, and what looks like a Chao Garden mini-game involving Cream and Cheese.

The site claims that Sonic Advance 2 will be "6 times" as big as Sonic Advance. Cream is described as a "beginner" character.

You can also see a VERY impressive video there. The video's character select screen seems to confirm that there are exactly 4 playable characters in the game. It's only up at certain times of day. The video reveals some very interesting stuff, so be sure to check it out.

According to Sonic Team's page, the game will be released on December 19, 2002, in Japan. ST again promises a game 6 times as large, and boasts "The fastest speediness in history."

Also from Sonic Team's page:

"In each character, it is an original action a large number addition! It is new. [Trick action] ‚ðŽg‚¦‚Î -- it can go apparently, and it does not go to the place which is not likely to be or does not take -- or [ that a hand also reaches a ring ]"

This broken translation COULD mean that each character takes their own path in each level, according to your interpretation.

Source: Sega Of Japan