I almost missed an exciting discussion (darn you work).
In regards to shoe-ins, I consider shoe-ins as someone with absolutely nothing going against it and everything going for it. When it comes to shoe-ins, the only one I can think of is Mewtwo.
Mewtwo has everything that other potential additions wish they had; massive popularity (likely most wanted internationally), important to it's respective franchise, easily fulfills Sakurai's four company, planned for all three Smash games, and the interest of both Sakurai and the Pokémon Company. No doubt will both Sakurai and the Pokémon Company will want Mewtwo in Smash, so this addition is a no brainer.
Anyone else that isn't a veteran (and I would even say Ike, Sonic and Snake are at risks of being cut, although I do lean towards them staying) could miss out one way or another. Whenever that is lack of popularity (Toad), importance (Geno), uniqueness (Chrom), potential issues with game play (Little Mac), becoming a victim of priority (Roy in Brawl), or messy licensing issues (any third-party), we can't really say that there are any other shoe-ins. There are highly likely additions, but aside from Mewtwo, arguing a shoe-in position can't be done.
But as for characters I strongly believe will be in, I also feel as King K. Rool, Takamaru, Palutena, Shulk, Little Mac and Ridley are highly likely if not almost (key word, almost) guaranteed to be in. I also feel as Mega Man has a really good chance to be in if we get a new third-party (since he stands out from the rest of the potential additions), but third-party status keeps him from being an highly likely addition.
So updating my tier system for likelihood on candidates I find realistic:
S Tier (Shoe-in): Mewtwo
A Tier (Confident, but not guarantee): King K. Rool, Takamaru, Palutena, Shulk, Little Mac, and Ridley
B Tier (Good odds, but not safe bets): Mega Man, Roy, Isaac, Dixie Kong, and Mach Rider
C Tier (Less likely, but still plausible characters): Samurai Goroh, Chrom, Toad, Bowser Jr., Saki Amamiya, Krystal, Ghirahim, and Professor Layton