A couple of interesting news items on TotalVideoGames.com add speculation to the GTA: San Andreas features list.
I've dug up a couple of posts (1, 2) which hint at what to expect in GTA:SA. In summary:
They don't quote the source of this info, so take it all with a pinch of sodium chloride.
- You can challenge other drivers to a highway race
- They've signed up artists 50 cent, Justin Timerlake and Happy Mondays for the soundtrack
- 'Much larger' city to Liberty and Vice.
- Online multiplayer mode will be a major selling point.