Here, I talked about who is likely to get the boot. I'm basically saying the same thing here, but I will also talk about who will be coming back, or who will likely be coming back.
Definitely Coming Back
Donkey Kong - He's only one of Nintendo's MOST WELL-KNOWN characters. There's no way in Hell he's going anywhere.
Yoshi - Another one of the original characters, Yoshi is another shoe-in. He's one of Nintendo's most popular characters, so he's not going anywhere.
Captain Falcon - Main character of a semi-major series. Hell, he even had a cartoon a few years back.
Luigi - See Yoshi and DK.
Peach - Major, major character in the Mario series. Was going to be in the original SSB, apparently. Not going.
Bowser - I really doubt they're going to cut their biggest villain. BTW, he was going to be in the original Smash as well.
Ganondorf - Main villain of Nintendo's second biggest series, and wanted by an incredible number of people. When Sakurai mentioned that some characters would be 'remade', G-man was the first one I thought of.
Pretty Good Chance of Coming Back
Ness - The only reason he's not 'definitely' coming back is because Sakurai mentioned on the Japanese Melee site that he wanted to include the main character from Mother 3, which was cancelled back then but actually came out last year. Still, Earthbound is the series connection to the US, and I think fans from both sides of the world will enjoy it if he appears alongside a Mother 3 alumni.
Jigglypuff - Popularity saved it in Melee, I see very little reason why popularity can't save it this time. Still not definite, though.
Zelda - Zelda seemed like a weirder choice, but she was a fun character nonetheless. I see no reason to cut her, but I can't say I'm 100% sure she'll be back.
Marth - He's the original lord of Fire Emblem, and the fan-favorite in Japan. Hell, there's a stage that might be from his game already shown, so he might become a default character this time.
Mewtwo - He's a popular Pokemon with cool abilities. He's been the focus of TWO movies. I'd say that's a pretty good qualifier.
Okay Chance of Returning
Ice Climbers - As per my comments on the "Most likely to leave" post, I want the Ice Climbers to come back, maybe to keep their role of early NES game rep. But still, there's a good chance they might not be needed anymore.
Mr. Game & Watch - I love Mr.G&W. He's so awesome on so many different levels of awesomeness. Still, his original role of 'surprise final character' might not continue, and I'm just not too sure 'rep'ing one of Nintendo's earliest forays into portables' is a really strong argument to keep him.
Falco - He's a fan-favorite and one of the biggest characters in Star Fox. Even so, I can't help but feel he may lose his way if another SF character shows up.
Low Chance or Returning
Dr. Mario - My favorite character in Melee, but lord if he isn't the most uninteresting clone. Drop 'em in a heartbeat.
Roy - Completely interchageable with every other FE lord ever, and some are just plain more popular.
Pichu - Cute, but worthless.
Young Link - Unless they do something interesting, like making him the YL from Wind Waker, I don't see much point.
Sheik - Sadly, they may want to focus on Zelda by herself because of Twilight Princess. They might decide not to, as well.