Not nessecarily Smash 4, but I hope for some of these to be in it in the future. I'll explain why, too.
Keep these in mind:
Sakurai's "4 Rules of Smash" he stated himself:
1) The character’s inclusion must make people want to play the game. (Pretty much means they must be popular and/or important.)
2) The character must be unique.
3) The character must fit into the style of Smash Bros.
4) The character must contribute to the game balance. (This is pretty much up to the developers.)
* Sakurai has broken these before (those WTF characters, and clones) so we can't really hold this up as law, and he also mentioned for Smash 4 that he wanted the roster to be made of games of national release.
I will try my best to keep these in mind, but don't hold it against me.
Medusa- I want her mainly because she represents Kid Icarus just as much as Pit and Paletena does. IDK how Dark Pit made it in but not Medusa (I didn't even know Dark Pit even existed in until Smash). She was in it since the original and was heavily advertized for Uprising. As a bonus, for those who really care, adding her would both add a female and a villain to Smash, 2 birds with one stone.
From a gameplay standpoint, she has lots to work with from her source material. She is definitely not a clone of Paletena. She has shown great physical and magical powers. Moves shown include claw attacks, laying mines, teleporting, poisoning gas, making eyeball sentry things, and of course her seldonmly used ability to turn people to stone. Also, don't give me that "she's too huge" because Paletena can become that size if she wanted to herself. So we can assume Medusa can do that, too.
Ridley- I know it's been done to death, and unless Sakurai is trolling the shit out of us like he normally does, he's pretty confirmed to be a stage boss in the Wii U version, but the hope never dies. Metroid is a big staple for Nintendo, but only has one rep as a playable: Samus twice. It's upsetting to see Metroid stand in Smash so long without another rep. Ridley is the only reoccurring character to make sense as playable, along with Dark Samus who is more obsure to outsiders. Before ANYONE says anything, Ridley is NOT too big for Smash. Just shut it. Many games have shown Ridley being not much bigger than Samus, around as big as Bowser is in 4, if not just a little bigger. Besides, even if he was too big, they can shrink him down to fit as a player. They've taken liberties with size on characters mostly growing characters like Pikachu, who is 1'04 ft in canon, yet he is only a little shorter than 5'01 ft Mario. So if "Ridely is too big," than Pikachu, Olimar, Kirby, Meta Knight, and Jigglypuff should be "too small." Not for discussion, moving on.
As a moveset, he seems like a "generic dragon" like Charizard, but he has a lot of things that make him unique from him, as he has used not just claws, but he has a stinger tail and has used his talons in battle, and he has plasma breath, which is either balls of fire or as an energy beam (NOT like flamethrower.) Ridley has also shown the ability to camouflage, and harden, which increases his resistance and power himself up. He can also regenerate by eating people, so that is something unique there. Also, they can do something unique with his air game, like make him fly around but make him much more launch vulnerable while flying, but he gets better air moves, like the opposite of Mac (I heard this from a "rumor" leak.)
Issac- He's from Golden Sun, an RPG that sold pretty well worldwide, especially in Japan, and still has a large following in the west, but unfortunately, in Japan, his fanbase has grown rather tired. Regardless, he appeared as an assist in Brawl, and requests for him are still rather high.
You're probably thinking "Another Sword user," but that's where you're wrong. Yes, he uses a sword, but he has other unique properties. Mainly, he could use "psynergy" attacks in addition. Most of his Psynergy spells sre mostly made of Earth spells, of which NO other Smasher uses Earth attacks as a main thing. At most, you had an earthquake, or Charizard's Rock Smash (which is technically a Fighting move.) This alone gives him the ability to give him a unique moveset. If that isn't enough, he also has summoning spells, and he has the ability to equip "djinn," which affect Issac in various ways, like powering up his psynergy, and other such side effects. All these give Issac something unique that most other fighters don't have. For reference, Shulks only real notable trait is his Mumando.
Chorus Kid(s)- You might remember them from the "Famitsu Leaks," and I feel they dropped the ball not implementing them. I don't think they should function like the Ice Climbers, but rather, have one Chorous Kid while it "summons" the other kids during attacks, or even other characters and items from Rythem Heaven (similar to G&W.) I feel Rythem Heaven is underrepresented, and having an enemy is Smash Run really stands out as weird where every other enemy has a series that is in Smash Bros. (sans Subspace, and Bacura and Pooka, who belong to Pac-Man but are from different series, but Pooka does appear in some Pac games, and Bacura is an enemy that appears in other Namco games as an easter egg like the Special Flag.) Even weirder that they omly have 3 trophies, but it merits an enemy.
Dark Samus- You immediately think "A damn clone" but her phazon energy gives her tons of leeway for unique movesets. Sure, she looks the same as Samus, but I am confident that at most, you'd get a semiclone, which is like Toon Link or modern Ganondorf. But there is room for unique moveset. It is also the only other character from Metroid who would make sense. Too bad it's an assist, and if they did make it a character, they might go the Dark Pit or Lucina route, which would get me upset.
Zoroark/Zoura? I thought they were going to either replace Lucario with Zoroark, or have Zoroark be there along with Lucario (and Greninja.) Lucario made it in Brawl probably to rep the lastest movie "Lucario and the Mydtery of Mew" and I thought Zoroark was gonna fo the same and rep 5th gen, but there is unfortunately no 5th gen playable Pokemon, and Zoroark is a Poke Ball. So that is the only reason I listed Zoura as to replace him in a even less likely event it get chosen, but I will talk about Zoroark.
It's main selling point, in addition to claw attacks, is the potential of darkness-based projectiles like Dark Pulse, and Night Daze, in addition to things like Night Slash and Snarl, or maybe even Low Swerp, U-Turn, or Fury Swipes. Dark Pulse is a wave attack as opposed to a ball Lucario
and Mewtwo has, so that makes it different. Using it's ability Illusion in addition to all this would be amazing. Zoura would basically be the same to replace this, but smaller and lighter, but IDK any tilts.
IDK any 3rd or 2nd gen Pokemon to put in. Maybe Scizor for 2nd gen?
Impa- Why Impa? TBH, I'd rather have her completely replace Sheik and absorb her moveset rather than be her own character, as in terms of representing a franchise, I don't like it when one character appears twice. Since Impa is a Sheikah, and Sheik is impersonating a Sheikah, it would make sense to do that. Impa also appears in a lot of Zelda games, I think more than any other character who can fight as in other than those who are already playable.
Outside of being Sheik's replacement, I think the more likely route to implementing her would be to mix up between Sheik's ninja skills, and Zelda's magic tricks, which would give her a unique and interesting playstyle. But I wouldn't want carbon copy mixed moves, but original skills.
Honerable Mention, Takamaru- Was probably my most requested character for Smash. He is a samurai with ninja stars that stars in his own Japan-only game "Nazo no Murasame Jo" ("The Mysterious Murasame Castle") He also appeared as a guest in the Wii Samurai Warriors, and has a Nintendo Land minigame. He has an assist tropy now, and he is pretty much no longer on my request list because now they want to keep playable characters limited to national released games, and would apparently be a "generic sword user." So my hope for him has pretty much fallen off. But a small part of me still wants him badly.