I'm hoping that Young Link will make a re-appearance (though if anything, it'll be Skyward Sword's Link) and I'd like to see a re-appearance of MewTwo (though it'll probably be Zoroark). I also hope (but doubt) that Sora and/or Riku from Kingdom Hearts will be playable characters (why not? They used Sonic and Solid Snake for Brawl, and btw, can't wait for Dream Drop Distance!).

Several things I would love to see:

Haxorus as a playable character
Eggplant bombs as items
Mickey from Epic Mickey as a playable character
Dragonspiral Tower as a stage, like Spear Pillar was in Brawl, but with Zek/Reshi/Kyu

I'm sure I'll think of other stuff, but all that would be awesome.

I really hope they return Roy and add some other fire emblem characters. Also I hope they will add some golden sun characters, and give us some more custom stage things to work with. Oh and they should make dialga palkia giritina and raqyazua as playable characters.

Hopefully a sub in the Pokemon Trainer. I think that choice was terrible. They could've went with like a different region or something.

They should add a mode that is like subspace emissary but you can take different story lines depending on your choices.

I think being able to play as Leon from Resident Evil would be cool. Gosh I am just edging for a gameplay trailer of this. Something. Please Nintendo.

I don't have a specific list at the moment, but most are third party game characters, which include:

Sonic the Hedgehog (Keep him in ).
Shadow or Metal Sonic.
MegaMan (If they can add him in Street Fighter X Tekken, which we all regret, they can add him in this).
Dante from Devil May Cry.
Trainer Gold.
Zero (MegaMan series)

And that's all, Nya~. Yes, I know that 99% or all won't be added.

More earthbound characters and Lyn from Fire Emblem. But what would be awesome, Is a character creation feature. You shape your own model, color it, give it some moves and a final smash, and use pre-set voices (or use a microphone if you have one) that you think fits.

Well, I think adding Sonic is a no brainer, but what about Knuckles the Echidna or Shadow? I'd love to see Simon Belmont whip some butt in SSB4! I would like to see less gimicky stages and more cleverly laid out stages. i would like a more robust Online feature and a Call of Duty-esque leveling up system"

I just hope that the game has the characters that I use .
Oh and I really , really hope that the newest one will be better
than Super Smash Bros Brawl. It sucks that they wont be adding
too many new characters at all .

I'd like to see Young Link return. Think about it; he could use the Trans. Masks from Majora's Mask. His final smash would be Fierce Deity Link. Also Megaman, since he was the most requested for Brawl, besides Sonic. Also, some more Kid Icarus characters, like Magnus or Viridi(?). 

It's really hard to assume what characters will or will not be in. It depends on primarily what the developers want in terms of franchise, mainly Nintendo. Most new characters will relate more to what Japan wants rather than westerners.

IMO, I can easily see Lloyd Irving as a character, due to the fact he is the most renown character in the Tales of... series, both Eastern and Western. That and Namco is developing Smash this time AND the head of the Tales of... series is part of the next smash development team, which gives a greater chance of featuring a Tales of... character. I say Lloyd because he's one of the more popular character and essentially revolutionized the Tales of... series in the west, making the series that much more popular. Lastly, in the recent modern Tales games, the gameplay is fairly similar to that of smash, which can easily be thrown into a smash format.

I can also see Shulk from Xenoblade as another character. The Xeno series definitely has not seen some Nintendo action only until recent, but when blade hit the Wii, it was a top seller in both Japan AND EU. Also that and Namco also had some relations in the Xenosaga series as the publishers, with the developers "Monolith Soft"; the same Dev. Team who had made Xenoblade.

There are only just a couple of predictions I made and sadly a lot better ones than I've seen on other forums, even the main smash forums. There's a reason I'm never on those boards because it's a huge group of elitists and dealing with it is just not fun.

 I would like these characters as playable:
All Brawl characters (Save for Zero Suit Samus, I think there are better characters that could have used that character slot.)
Mewtwo (Pokémon) back from Melee
Ridley (Metroid)
Lyn and/or Hector (Fire Emblem) Although I am not sure how much they would differ from speed orientated Marth and power based Ike respectively.
Another F-Zero character, probably Black Shadow, although they would have to build a complete move set for him. If they did it with Captain Falcon, I don't see why not.
A separate Zelda and Shiek.

Character changes:
Generally more differences in clones, such as the Links and Space Animals. This includes final smashes.
I would like to have a completely new Ganondorf. Perhaps wielding the light sword, or even better, the Trident from the old games.
Get rid of the Gale Boomerang for Link, and replace it with the ordinary one.
Some sort of beam/skyward strike move. Remove the charge from the ground spin attack.
For Sonic, go the Wario route and have his Super Sonic form need to use attacks, makes it a little less cheap. Make his moves come out faster if weaker to balance, and replace one of the rolling attacks with something else.
Nerf Metaknight.

General Changes
No more tripping
More hit stun for proper combos

These are just a few things off the top of my head, I'm sure there are more that I could come up with after more thought

All I care is the there is at least 1 Fire Emblem character in the new games.  The characters are great and please bring Ike back.  He is awesome!

I think they need to make more characters from the sonic series,more characters from the earthbound series,more characters from the fire emblem series,and more characters from the metal gear solid series

All the characters of Melee and Brawl would be awesome, along with new ones. I'd love new characters like Ridley, Mega Man, Ryu of Street Fighter, maybe Knuckles, etc. There are so many characters that the list could go on and on. Although, I suppose that polishing the game's mechanics is more important than a huge amount of content, especially making every character close to even.