I think at this point the only pretty much guaranteed character is Ridley; there's likely choices like Mewtwo coming back, or Little Mac, but that's all just speculation. Ridley is the only shoe-in that is left, honestly.
Ridley and Palutena are the only new characters I'd put money on.
Something that's worth nothing: besides Pit and R.O.B. (retro characters), EVERY Brawl newcomer was relevant when the game was in the planning stages:
Wario - A long overdue icon who exploded in popularity since Melee with the creation of the WarioWare series.
Meta Knight - Also long overdue, appearing consistently in new Kirby games.
Zero Suit Samus - Zero Mission was a year old when Sakurai decided on the roster, and he wanted to increase the amount of women and Metroid characters.
Olimar - A Miyamoto creation; Nintendo's biggest new series on the GameCube.
Lucas - MOTHER 3 was in development, and he was supposed to be in Melee.
Diddy Kong - An overdue icon appearing consistently in DK games.
Pokémon Trainer - An iconic and unique character representing the year-old FireRed and LeafGreen remakes.
King Dedede - Yet another overdue icon.
Lucario - The face of Diamond and Pearl, which were in development at the time.
Ike - Starred in both Path of Radiance and the in-development Radiant Dawn.
Toon Link - Starred in Wind Waker, the first of two major Zelda releases since Melee.
Wolf - Appeared in the then-new Star Fox: Assault and the in-development Star Fox Command.
Characters like K. Rool don't quite fit those parameters for SSB4.