My guess is that all(or close to all) the currently listed, non-3rd party-characters will be our starting roster.
As for Zelda... I wouldn't care if she had Sheik or not. Just because Sheik was High tier in SSBM(well, for most of her lifetime in melee, she was top, but only recently high), doesn't mean she would be high or top in this game. For example, Bowser, the 4th worst character in Melee, has been given so many upgrades that he'll probably be mid-upper mid in Brawl(read Gimpyfish's character information on
www.smashboards.com, it has in-depth information on each character, based on Gimpy's experiance with the demo at E 4 All.)
Shame about Ike though... He'll probably end up bottom tier.