There just aren't a lot of worthy characters left.
There are plenty of worthy characters to choose from:
Pac-Man: One of the biggest gaming icons, and the perfect second third-party character for this next Smash Bros., with Namco Bandai developing it and all. Pac-Man has had a great relationship with Nintendo, with Miyamoto even having designed a Pac-Man game himself (Pac-Man Vs.). Also, let's not forget that Pac-Man has teamed up with Mario on multiple occasions before! And with the upcoming new Pac-Man games and the recent debut of his own TV show, the timing couldn't be better!
Mii: Miis have become a HUGE staple to Nintendo since their debut on Wii, and they have since been featured in many, many games, from Wii Sports to Mario Kart to Pilotwings...and the list goes on and on! It's only natural to include them in Smash Bros., too! It would add some more customization to the game, as well.
Little Mac: Now, this guy is just overdue to be in Smash Bros.! He didn't quite make it into Brawl, but after the new Punch-Out!! game for Wii, his time has come! I'm guessing the main reason why he hadn't been a top pick for Sakurai before was because...well, what exactly would his move set be? He doesn't really do much besides...punch. But if Sakurai can make a move set for Captain Falcon from scratch, he can figure out something for Little Mac.
Mewtwo: He needs to return. It was a sin to cut him in the first place! He has a unique move set, and there's no better time for Mewtwo's Return with Mega Mewtwo from Pokémon X and Pokémon Y being in the spotlight right now. (I'd love to see Mega Mewtwo represented in the game somehow, preferably as a Final Smash or something similar, but not as a replacement for normal Mewtwo.)
Takamaru: I believe Takamaru falls in line with characters like Pit. He's not exactly relevant, and he doesn't have a big franchise behind him, either, but he is a fondly remembered retro character, and I believe he stands out among the rest, as Pit did before him. He's been getting more recognition lately, too, with the Murasame Castle in Samurai Warriors 3, the Takamaru's Ninja Castle attraction in Nintendo Land, and the expected upcoming Virtual Console release of The Mysterious Murasame Castle outside of Japan for the first time.
Chrom: Fire Emblem is tricky when it comes to potential new characters for Smash Bros., much like Pokémon, because there are new main characters with every new game. But why do we need a new character just because there have been new games? Had it not been for Fire Emblem Awakening's CRAZY success and popularity, I wouldn't believe that a new Fire Emblem character would even be considered. Marth and Ike alone are perfect characters to represent Fire Emblem, because they're the two lords that have appeared the most in the franchise. Chrom doesn't have multiple appearances behind him, but he is the main lord from the fastest selling and most popular (worldwide, that is) Fire Emblem game yet. I can see him being included because of that, but not at the cost of losing Ike. I think they'll either add Chrom as a third Fire Emblem character, or just represent Fire Emblem Awakening in other ways (like the Arena Ferox stage in the 3DS version).
Ridley: Metroid could use another character, and Ridley is the only good choice for that character! I don't think he's a shoe-in, but I can definitely see it happening. He doesn't have to be "too big!"
Waluigi: Now, there are a few different potential choices for a new Mario character. Personally, I believe it comes down to either Paper Mario or Waluigi, and I think Waluigi would be the better choice of the two (not that Paper Mario wouldn't be awesome, too!). I think Waluigi deserves a little more. Mario, Luigi, and Wario have all had their time to shine, so when will it be Waluigi's turn? He's such a mysterious character because so little is known of him and his abilities, and because of that Sakurai could do a lot of really interesting things with his move set! It might even pave the way for a Waluigi game, or at least an appearance in a main series Mario game! At the very least, it would complete the set of Mario, Luigi, Wario, and Waluigi!
King K. Rool: Smash Bros. could always use more villains, and King K. Rool is one of the best you can choose! He may not be super relevant today (his last appearance was Mario Super Sluggers in 2008), but he's still the main villain of the Donkey Kong franchise, and he'd be a good choice for a third Donkey Kong character should there be one. I can see Dixie Kong's potential, too, but I'd think that we'd get a villain before another protagonist.
Palutena: I don't think that Kid Icarus really needs a second playable character, but if it gets special treatment because of Sakurai's involvement in the franchise, then Palutena is really the only option. She's the Zelda of Kid Icarus. She's the titular character of the franchise in Japan, and she's the only character besides Pit who has appeared in all three Kid Icarus games. Choosing any other Kid Icarus character over her would just be silly.
Shulk: Now, Shulk's only been in one game, I know. One extremely well received game, which will be getting a sequel on Wii U. We know he's not just from a one-off game, as the Xenoblade franchise is clearly still an active one. He's one of the more logical choices for a character from a relatively new Nintendo franchise. I don't know how likely he is, but I wouldn't be surprised to see him make the cut.
Isaac: I'm not entirely sure what to think about Isaac. Golden Sun is one of the few Nintendo franchises that I have yet to play, so I don't know much about his character or anything, but I do know that his games were very popular and well received during the GBA era. His franchise only spans 3 games, but the most recent Golden Sun game for DS is only a few years old, so I don't think he's too irrelevant. I don't think he's incredibly likely, but I can see it happening. Of all the other smaller Nintendo franchises like his out there, I think Isaac is one of the best picks for Smash Bros.
Dillon: I'm kind of scraping the bottom of the barrel now, and while I wouldn't really expect a character like Dillon making the cut, I can see it happening if only to represent eShop games. There hasn't ever been a character or any other form of representation in Smash Bros. from a franchise that's been exclusive to WiiWare/DSiWare/eShop. Granted, that wasn't really possible before since WiiWare wasn't even around until after Brawl's release, but now that Nintendo has had a presence in the digital side of things for awhile, perhaps they'll want a character to represent that side of Nintendo's catalog? If so, I think Dillon would be a fine choice.
Tingle: If another Zelda character were to get into Smash Bros., it would have to be tingle, in my opinion. Aside from Link, Zelda, and Ganondorf, he really is the next biggest character in the franchise. He's even got his own spin-off games! He's the only other Zelda character that has ever been playable aside from Link himself (and Zelda, if you count her role in Spirit Tracks). Not to mention he's quite popular (maybe not so much in North America...but still). He'd also be the perfect excuse to reference Balloon Fighter in his move set, since he does everything Balloon Fighter does and more, and is more of a character, too. One problem, though... They've already done that with Villager. So, because of that, I think Tingle's chances are very slim now, unfortunately. But I still don't see any other Zelda character getting in the game, so if we don't get Tingle, I don't think we're getting anyone.
That pretty much concludes the characters that I think have any kind of chance of making it into Smash Bros., though. If anyone other than any of the characters I listed above make the cut, I'll be truly shocked. Mega Man and Villager were on my list of potential characters before E3 along with the characters above, and it was no surprise to see them confirmed! The only big shocker to me was Wii Fit Trainer (but, really, did ANYONE expect her?). It makes perfect sense now, of course, but I'd be very surprised if Sakurai manages to pull off another unexpected (but very appropriate) character like her.