This is my prediction list for future Smash Bros. games.

Possible future Smash Bros. Characters:

From the Street Fighter Franchise

~ Dan Hibiki (It's hard to resist having the epitome of a Joke Character in Smash Bros)
~ Guile
~ Ken Masters
~ M. Bison
~ Akuma

From the Sonic Franchise

~ Shadow the Hedgehog
~ Knuckles the Echidna
~ Amy Rose
~ Metal Sonic
~ Dr. Robotnik (With Eggman Nega as a skin)
~ Blaze the Cat

From the Mega Man Franchise

~ Dr. Wily
~ Bass
~ Roll
~ Protoman
~ Mega Man X
~ Zero

From the Mario Franchise

~ Waluigi
~ Geno
~ Smithy
~ Mr. L
~ Dimentio
~ Fawful
~ Paper Mario
~ Rawk Hawk
~ A Goomba (Just for the pure hilarity of it actually winning)

From the Pokemon Franchise

~ Grovyle (Pokemon Mystery Dungeon: Explorers) (Mega Sceptile as Final Smash)
~ Shadow Mewtwo (Pokken Tournament)

From the Metal Gear Franchise

~ Solid Snake as a returning veteran
~ Big Boss
~ Raiden

From the Final Fantasy Franchise

(The Final Smashes of the characters from Final Fantasy I will be temporarily turning into their canon upgraded versions)
~ Black Mage (Final Fantasy I)
~ White Mage (Final Fantasy I)
~ Tifa Lockhart
~ Sephiroth
~ Red Mage (Final Fantasy I)
~ Warrior (Final Fantasy I)
~ Thief (Final Fantasy I)
~ Black Belt (Final Fantasy I)
~ Kefka Palazzo

From the F-Zero Franchise

~ Samurai Goroh
~ Black Shadow

From the Metroid Franchise

~ Dark Samus
~ Ridley
~ Fleet Admiral. Castor Dane

From the Kirby Franchise

~ Galacta Knight
~ Dark Matter (Giant eye form as Final Smash)
~ Knuckle Joe

Other Franchises

~ Scrooge McDuck (DuckTales video game)
~ Sora (Kingdom Hearts)
~ King Mickey (Kingdom Hearts)
~ Mickey (Epic Mickey)
~ Shantae
~ Shovel Knight
~ Isaac (Golden Sun)
~ Isaac (Binding of Isaac)

~ Quote (Cave Story)

~ Curly Brace (Cave Story)

~ Abraham Lincoln (Code Name: S.T.E.A.M.)

~ S.T.O.V.E.P.I.P.E (Code Name: S.T.E.A.M.)

Assist Trophies

~ The Star Spirits (Paper Mario)
~ The Crystal Stars (Paper Mario: The Thousand-Year Door)
~ A Time Gear (Pokemon Mystery Dungeon: Explorers)
~ A Game Over
~ A BSOD
~ A Reset Button
~ Big the Cat
~ Paper Bowser
~ A Mystery Box (Paper Mario Series)
~ One of the Special Attacks from the Mario and Luigi series
~ Missingno.

~ A Flying Pig

~ Splash Woman

Stages

~ Sky Tower (Pokemon Mystery Dungeon: Rescue Team)
~ Temporal Tower (Pokemon Mystery Dungeon: Explorers)
~ Citadark Isle (Pokemon XD: Gale of Darkness)
~ The Glitz Pit (Paper Mario: The Thousand-Year Door)
~ Smithy's Weapons Factory (Super Mario RPG)
~ Forest Maze (Super Mario RPG)
~ Booster's Tower (Super Mario RPG)
~ Bowser's Insides (Mario & Luigi: Bowser's Inside Story)
~ Neo Bowser Castle (Mario & Luigi: Dream Team)
~ Bowser's Castle (Theater Area) (Mario & Luigi: Bowser's Inside Story)

~ The Palace of Shadow (Paper Mario: The Thousand-Year Door)

~ The Overthere and Underwhere (Super Paper Mario)

~ X-Naut Fortress (Paper Mario: The Thousand-Year Door)

~ Standard Battle Screen Stage with Interactive Audience (Paper Mario: The Thousand-Year Door)

~ Courtroom (Ace Attorney)

~ Mario's Rainbow Castle (Mario Party)

~ Toy Dream (Mario Party 5)

~ Pinna Park (Super Mario Sunshine)

~ Space Land (Mario Party 2)

~ Waluigi's Island (Mario Party 3)

~ Clockwork Castle (Mario Party 6)

~ Pagoda Peak (Mario Party 7)

~ Goomba's Greedy Gala (Mario Party 4)

~ Magma Mine (Mario Party 9)

~ King Boo's Haunted Hideaway (Mario Party 8)

Stage Bosses

~ Perfect Chaos

~ Dark Matter (Pokemon)

~ Shadow Lugia

~ Mecha Bowser (Super Mario Sunshine)