This just in - we just learned that Sonic Adventure 2 will use a world map in order to progress from stage to stage. This marks a change from its predecessor, which saw players travel between Zone markers using an Adventure Field.
From the various previews we've seen, it doesn't seem like the world map is too obtrusive - gameplay in the Story Mode tends to flow seamlessly, with each level starting one after another. If that ends up being the case when the game is released, this world map could be a backup instead, used for level selects in other menus perhaps.
The more we hear about Sonic Adventure 2, the closer it sounds like Sonic Team has refined the best elements of its predecessor while cutting the weaker parts entirely.