Copying its content over because the dumbarse GameFAQs mods delete all "leaks."
Pardon if the English I use is not the best.
Before I go on and discuss what I know, I am not Sakurai. I am not claiming to have worked on the game, though I have seen some production sketches and even heard about the game progress over dinner with a few of my friends who work on the game. If you do not wish to be spoiled or disappointed, I suggest to stop reading. Now here is what I know for sure.
New Items:
Barrel Cannon from Melee and Donkey Kong
Tiki's Tear, which increases damage given by 1.1X and damage taken .9X. From Fire Emblem Kakusei
Baby Yoshi that blocks projectiles, can wander off stage and can be KO'd. From New Super Mario Brothers U
New Stages Include
Lor Starcutter, which is a peaceful stage that can tilt due to Landia and Magalor Fighting over the Master Crown. This stage is on the Wii U.
Pokemon World Tournament, in which certain Pokemon like Garchomp and Tyranitar will battle. They become a stage hazard when Mega Evolution is in play though! This stage is on the 3DS.
A Chimera Factory stage from Mother 3, in which different chimeras roam, waiting for snack! This stage is on the 3DS
An 8 bit version of Ice Climber, in which players can destroy platforms and use enemies to KO opponents, This stage is on both Wii U and 3DS
Prison Island from Xenoblade, in which Metal Face appears in the background, and is fought in adventure mode. The music is to die for on this stage. This Stage is on the Wii U
No Melee Stage is returning unless you count Battlefield and Final Destination.
New Assist Trophies
Sallya, who uses Dark Magic to damage the opponent and heal her summoner. From Fire Emblem Kakusei
Tails, who can save a summoner from being knocked off only once, but can be KO'd himself.
Magnus from Kid Icarus, who uses his sword to do two strikes, but can be dodged easily.
Character Details
My Unit from Fire Emblem Kakusei is a newcomer. My Unit uses a unique stance change moveset, in order to diversify the Fire Emblem newcomers. There are costumes for both a male and female My Unit. The final smash involves Grima, though Grima is also an adventure mode boss.
Shulk, who uses the Monodo to strengthen his own attributes in battle, and is about as fast as link normally. He is the most recent character to be added into Smash Bros.
Ridley, who Sakurai added in order to please Western Fans. He is the largest character in the game, a bit taller than Bowser. He focuses on aerial combat, and has a hover much like Peach's, His final smash is Meta Ridley
The Pokemon Lineup is Mewtwo, Lucario, Jigglypuff, Pikachu, and Pokemon Trainer, who reprises his pokemon.
Discussion on Snakes return are vague. The possibility of replacing Solid Snake with Naked Snake is there...
There are no other Mario characters besides Rosalina added, though Dr Mario is a Mario Skin...
The only cut characters are Pichu, Young Link, and Roy. Snake is possible at this point, if only to be replaced by his father, who would be called Snake.
Keep in mind that this is all i know, and this is a throwaway account.
I think it's false because Snake's return should in no way be vague this late in development, given his third party nature, though maybe that's just the guy's alleged poor English coming into play. It's at least worth noting that both this and the PackedMeGloves leak from a few days back had Lorr Starcutter as a new stage (this leak loosely matches the description of the previous one of it being a variation on Corneria; it's peaceful, just tilting somewhat).