For so long, people have been stating who they want playable, theorising who will be playable and who they think should be playable Super Smash Bros. 4, well before it was even announced at E3 2011. To me, this is so infuriating, because of only a small percentage of people are being realistic about it.
For a character to be playable, they must have originated from a video game and appeared on a Nintendo console. Despite this, I have seen too many people around the internet show genuine interest in having characters such as Goku and Mickey Mouse playable, as if they had a chance. People are like "omg, i'd be so cool!".
There is also an indirect third rule. A playable character must be a large part of their franchise, as in, a staple. Think back to Melee, where Dr. Mario was a playable character. He was removed from the series and not in Brawl. Why? Why the do you think!? He didn't even compare to the main Mario cast, that consists of Mario, Luigi, Peach and Bowser, who are some of the most popular characters in all of video games. You can't have the two Mario Brothers, Princess of Mushroom Kingdom, Koopa King, and then a Doctor spin-off character with, like, 3 games. That is too ridiculous. Characters like Yoshi, Captain Falcon and Samus are alone because nobody else compares to them. There actually isn't many more first-party characters that are important enough to fill up spots on the character roster.
The main importance on whether a character should be playable or not is influence. Link, Zelda and Ganondorf are undoubtedly the most important characters in the Zelda series. Characters such as Skull Kid, Vaati, Ghirahim, Impa, Demise and Groose are not up to par with the mains. They're more suited towards an Assist Trophy role in the game, like Tingle, if anything.
Characters can be put in for several different reasons. Mr. Game and Watch, R.O.B, Ice Climbers and Pit were put in for nostalgic-sake (and in Pit's case, to reboot the series, in a sense), Snake was put in due to a developer request, Jigglypuff was put in SSB64 to put the roster at an even twelve character and because of her extremely similar model to Kirby, and the most interesting case, Sonic was put in for fan service and nothing more. It has been stated the next game will not include too many new characters, so the likelihood of the developers repeating any of these actions is unlikely and would not be a wise move, anyway.
Rayman, Megaman and Bomberman are very possible inclusions that could fill up a spot or two.