Luigi, Peach, Donkey Kong, Ice Climbers, ROB, Pikachu, Jigglypuff, and Mr. Game and Watch need better Final Smashes. Their current ones suck!
Pikachu has one of the best smashes in the entire game (Probably only behind Sonic, Snake and Marth, in no particular order). One of the few that allow you to KO a character multiple times if you know what you're doing. DK's final smash isn't that bad, you just need a sense of timing with the beat of the drums. Game and Watch's final smash is also a decent one, although it is hard to use at first. Luigi and Ice Climber's Final Smashes aren't designed for directly killing, but both provide a way for easy kills for yourself. I agree the rest are bad.
Characters I want to see in SSB4:
Pokémon Trainer without stamina (Being able to to play as any of the three without worrying about them getting weaker allows better strategy. Stamina holds back Pokémon Trainer from being an otherwise great character)
Mewtwo (My favorite character in Melee, and only non-clone cut in Brawl. Really sad to see him go, even if he sucked competitively. For the level I played, he was awesome)
Ganondorf with his sword (Stop it with the clones. Ganondorf being one of Captian Falcon is just wasting his potential of him. The only clone I liked more than the original was Roy, which, speaking of)
Roy (Love him as a character, but he needs to be more than just a fire based Marth with his sweetspot in a different place. If they Luigified him, that would be amazing)
Little Mac (See last post)
Ridley (Size is not an issue when he was Samus's height in the first Metroid. Plus, he would be a completely original character unlike Dark Samus)
Ray Mk. II (I second Martin's sentiments, even if Ray Mk. III was the one that appeared in Brawl. II's more human-like design makes me prefer him over III though)
Vaati (Honestly, I think we have enough Zelda characters already, but I'd rather have an original character over another Toon one. Speaking of which, if we must have a second Link, Young Link would be prefable. Majora's Mask beats Wind Waker any day of the week)
Geno (Square owns the rights to him, so I doubt it will actually happen. Still, I think he might have a chance considering everyone who ever played Mario RPG wants him in, including me)
Samurai Goroh and Black Shadow (More F-Zero characters in general are good, as the only series from the original eight with still only one representative, and they are the two main characters besides Captain Falcon. Samurai Goroh's assist trophy could be the basis for an aggressive yet unprecise sword user, and Black Shadow could possibly take over current Ganondorf if he finally gets a unique moveset)
Snover (I CAN DREAM
)
That's about it for my suggestions for now