One important thing to keep in mind: Every single Brawl newcomer was featured in a game since the release of Melee and the release of Brawl with the exception of Retro characters. Sakurai will never include old characters within an existing series. Let's take a look:
Pit: Retro
R.O.B: Retro
Wario: Featured in Wario World, Super Mario 64 DS, Warioware series, Spinoffs, etc
Zero Suit Samus: Featured in Metroid: Zero Mission, Metroid Prime 2 and 3, Metroid Prime Hunters
Ike: Featured in Fire Emblem Path of Radiance and Radiant Dawn
Pokemon Trainer: His Pokemon are featured in Pokemon FireRed and LeafGreen
Diddy Kong: Featured in Donkey Kong Country and Donkey Kong Country 2 for GBA, Diddy Kong Racing DS, Spinoffs, Donkey Konga, etc
Metaknight: Featured in Kirby: Nightmare in Dreamland, Kirby Air Ride, Kirby Canvas Curse, Kirby Squeak Squad
Snake: Featured in Metal Gear Solid 3, 4, Acid, etc
Lucas: Featured in Mother 3
Sonic: Featured in Sonic Heroes, Sonic 2006, Sonic and the Secret Rings, etc
King Dedede: Featured in Kirby: Nightmare in Dreamland, Kirby Canvas Curse, Kirby Squeak Squad
Olimar: Featured in Pikmin 2
Lucario: Featured in Pokemon Diamond/Pearl/Platinum
Toon Link: Featured in Wind Waker, Phantom Hourglass
Wolf: Featured in Star Fox: Assault
The point of this is: Sakurai will not pick characters that have become less relevant by not appearing in games since the last installment in the Smash series. For whatever reason he left out the characters he did for a reason in Brawl, and he's not going to add a character in Smash WiiU if they haven't proven to be any more relevant by not appearing in a game since (except for Retro characters for obvious reasons).
Therefore, a short list of popular characters that we can confidently say have no chance of being in SSBWiiU:
-Geno
-King K. Rool
-Dixie Kong
-Midna
-Vaati
-Mewtwo
-Plusle & Minun
-Meowth
-Sylux
-Dark Samus
-Saki
-Krystal
-Samurai Gorah
-Black Shadow
-Lyn
-Hector
-Roy
-Sigurd
-Micaiah
-Ray (Custom Robo)
-Lip
-Any Mother character
-Any Advance Wars character not from Days of Ruin
The Pokemon are iffy, but I included them because none of them are part of the "National Dex" in any of their games, and therefore I can't really say they are featured. Even Isaac is at risk due to his small role in the Golden Sun sequel.