Though with contribution from similar threads, I have picked out an ideal character roster for Super Smash Bros. Rumble, with Final Smashes for added flavor.
Veterans:
Bowser (Mario)
Final Smash: Kamek flies in and makes Bowser giant, who jumps into the background and rams the stage, smashing everything that touches his silhouette (reflecting his appearance in Yoshi's Island).
Captain Falcon (F-Zero)
Final Smash: In the style of F-Zero stage effects, a stream of race-jets fly in, smashing all opponents they touch.
Donkey Kong (Donkey Kong)
Final Smash: The current one - a pair of congas that emits sonic shock-waves when played by Donkey Kong - is fine as long as it has better range and/or damage (reflecting his appearance in Donkey Konga).
Fox, Falco, and Wolf (Star Fox)
Final Smash: The current one - summoning and entering the Landmaster - is fine as long as the stage hit detection isn't so jumpy.
Ganondorf (Zelda) with Ike's move-set
Final Smash: The current one is fine - Ganondorf transforms into Ganon and charges off the stage.
Jigglypuff (Pokemon)
Final Smash: Jigglypuff sings for a few seconds, inducing the sleep state for all opponents. Jigglypuff then pulls out a black marker, temporarily making all of his/her smash attacks more powerful (reflecting his/her appearance in the anime).
King Dedede (Kirby)
Final Smash: I don't know dick about Dedede outside of SSBB, but his current Smash seems fine - summoning a bunch of randomly-spawning Waddledees, Waddledoos, and spiky balls with eyes, which run off the stage and knock-back every opponent in their paths.
Kirby (Kirby)
Final Smash: The current one - Kirby pulling out a cauldron, which sucks up nearby opponents, cooks (deals damage to) them, smashes them back out, and spawns items if they are enabled - is fine as long as its range and item-spawning are turned down.
Link (Zelda)
Final Smash: Spinning blacks squares fly up from under the stage, smashing all opponents they touch (reflecting his appearance in Twilight Princess).
Luigi (Mario)
Final Smash: Luigi pulls out the Ghost Vacuum which, as the player retains movement control, sucks up opponents it's pointed at, deals periodic damage to players inside of it, and can spit them back out as smashing projectiles (reflecting his appearance in Luigi's Mansion).
Mario (Mario) with drastic move-set alterations
Final Smash: Mario eats a Mega Mushroom, grows giant, and stomps around the stage at the player's will, smashing everything he touches (reflecting his appearance in New Super Mario Brothers).
Meta Knight (Kirby)
Final Smash: I don't know dick about Meta Knight outside of SSBB, but his current Smash seems fine - making the game slow down, the screen go black, and smashing everyone in certain places relative to himself.
Mr. Game & Watch (Game & Watch)
Final Smash: The current one - transforming into a giant squid Landmaster-style - is fine as long as the attacking vs. not-attacking state of tentacles is more obvious.
Mewtwo (Pokemon) with Lucario's move-set
Final Smash: Mewtwo summons clones of his opponents which attack their clone-ees Assist-Trophy-style (reflecting his appearance in Pokemon: The First Movie).
Ness (Earthbound/Mother)
Final Smash: I don't know dick about Ness outside of SSBB, but his current Smash seems fine - calling down "PK Thunder", a barrage of large energy balls.
Olimar (Pikmin)
Final Smash: I don't know dick about Olimar outside of SSBB, but his current Smash seems fine - flying up a rocket, then crashing down on his opponents - as long as the camera stays on the other players.
Peach (Mario)
Final Smash: Peach creates a word bubble in the middle of the stage, filled with bombs that drop out of it (reflecting her appearance in Mario & Luigi: Superstar Saga).
Pikachu (Pokemon)
Final Smash: Pikachu jumps onto a cloud above the stage and, at the player's will, flies around zapping the other players.
Pit (Kid Icarus)
Final Smash: I don't know dick about Pit outside of SSBB, but his current Smash seems fine - summoning a group of other angels to ram his enemies - as long as the angels come in greater quantity.
R.O.B. (NES accessory)
Final Smash: The current one is fine - R.O.B. acquires a boost of power, allowing it to fly around the stage, blasting opponents with laser beams.
Samus/Zero Suit Samus (Metroid), though Zero Suit Samus should be separate, and thus also a different character, such as Nova from the canceled StarCraft: Ghost.
Final Smash: The current one - firing a large energy beam, then transforming into Zero Suit Samus - is fine as long as the power level is turned down. It could also do without the character switch.
Snake (Metal Gear)
Final Smash: I don't know dick about Snake outside of SSBB, but his current Smash seems fine - jumping onto and hanging from a ladder dangling from a helicopter, and firing at his opponents with a grenade launcher from this position.
Squirtle/Ivysaur/Charizard (Pokemon)
Final Smash: Squirtle joins Wartortle and Blastoise (summoned from Pokeballs) in spraying a wave of water which pushes opponents away. Ivysaur joins Bulbasaur and Venusaur in tunneling vines under the stage, grabbing opponents with said vines, and pulling them to the 'saur trinity, where they are then stomped on. Charizard joins Charmander and Charmeleon in launching a large smashing Fireblast across the stage.
Sonic (Sonic)
Final Smash: The current one is fine - Sonic transforms into Super Sonic, a golden form in which he can fly, move even faster, and smash opponents by touching them.
Wario (Mario)
Final Smash: The current one is fine - Wario dons a purple mask and cape which make him faster, stronger, and able to rise aerially by attacking.
Yoshi (Mario) with drastic move-set alterations
Final Smash: Yoshi enters egg form, where he is then pulled by Super Baby Mario, a player-controlled character who can run, jump, scale walls, and smash things by touching them (reflecting his appearance in Yoshi's Island).
Zelda/Sheik (Zelda), though Shiek should be separate, and thus also a different character, such as Ryu Hayabusa from Ninja Gaiden.
Final Smash: The current one is fine - Zelda fires an arrow made of light from a similarly composed bow.
Newcomers:
Alex Mercer ([Prototype])
Final Smash: Alex whips out the claws, stabs into the ground, and stabs outward from the ground with a barrage of larger claws.
Alien Hominid (Alien Hominid)
Final Smash: The Hominid attempts to jump onto an opponent in the style of Diddy Kong's side-B attack. If successful, he instant-kills that player by biting their head off, then jumps off their body, at which point his player may repeat this process by successfully landing on another opponent.
Banjo/Kazooie (original/'Tooie form, Banjo-)
Final Smash: Banjo summons Mumbo, who transforms all opponents into still-mobile but attack-less pumpkins (reflecting their appearance in the original Banjo-Kazooie).
Big Daddy (Bouncer, Bioshock)
Final Smash: The best thing I can think of "he drills someone".
Colored knights (Castle Crashers), with elemental power determined by costume
Final Smash: The other four knights (I'm counting the gray knight here) are summoned Assist-Trophy-style to fight with you.
Diddy/Dixie Kong (Donkey Kong) in the style of Ice Climbers
Final Smash: Rambi charges across the stage (disappearing after a set time if caught between walls), smashing all opponents in his path (reflecting their appearance in Donkey Kong Country 2).
Generic Firebat (StarCraft)
Final Smash: The Firebat visibly uses a Stim Pack, dealing 10% damage to himself but drastically (and temporarily) increasing his speed and damage output.
I-No (Guilty Gear)
Final Smash: I-No plays several power chords on her guitar, sending sonic waves at her enemies.
Isaac (AKA Robin, Golden Sun) with Marth's move-set, possibly with Ivan and Garet (AKA Gerard) as drastic fast/weak and slow/strong versions
Final Smash: Isaac calls down Megiddo (a small but fire-resistant meteor) immediately in front of himself (reflecting his appearance in The Lost Age). Ivan and Garet use Spark Plasma and Pyroclasm respectively.
Knuckles (Sonic)
Final Smash: Knuckles becomes flying and golden like Super Sonic but slower and with a large attack radius.
Kratos (God of War)
Final Smash: Kratos unleashes the Army of Hates, a group of projectiles that chase after opponents and smash them on contact (reflecting his appearance in the original God of War).
Jak/Daxter (Jak)
Final Smash: Jak transforms into Dark Jak, a giant form with more damage output and one attack replaced by a ground-based shock-wave (reflecting their appearance in Jak II).
Master Chief (Halo)
Final Smash: Master Chief rocket-jumps off the screen, and falls back down in a Wraith tank, an alternate form similar to the Landmaster but with energy turrets instead of barrel rolls, a delay and arc in the energy blast's movement, and a moon-jump instead of full flight.
Mega Man (Mega Man/Rockman)
Final Smash: I have no idea what Mega Man's acquired powers are. Maybe his Final Smash could be that giant missile barrage from Marvel Vs. Capcom.
Rash (Battletoads), or both Pimple and Zitz as strong vs. fast versions respectively.
Final Smash: Rash jumps into the air with a propeller back-pack. In this form, he can fly in full two dimensions, hack away with a big white stick, and launch himself at enemies as a spinning disc (reflecting his appearance in the Battletoads arcade game (AKA Super Battletoads).
Ratchet/Clank (Ratchet & Clank)
Final Smash: Clank transforms into Giant Clank, a giant form with powerful punching attacks and missiles (reflecting their appearance in Going Commando).
Sarah Kerrigan (Queen of Blades form, though a Samus/Zero Suit Samus motif could work, StarCraft)
Final Smash: Sarah Kerrigan casts a Psi-Storm upon the stage, an area-of-effect attack which rapidly damages those inside it.
Tails (Sonic)
Final Smash: Tails creates a whirlwind by spinning his tails, sucking in and smashing out opponents.
Zeratul (StarCraft)
Final Smash: Tassadar appears, who merges with Zeratul to enter Archon form, in which he attacks opponents with arcing lightning-like energy.