I fear that things don't look good for Ness. Lucas appears to have replaced him. I'll miss the little dude if he doesn't come back. If he does, it means there will be clones, but clones with fairly significant differences.
I'm certain Luigi is coming back. Like someone just said, Mario without Luigi? No way. I'm interested though, since Mairo has F.L.U.D.D, what will Luigi have?
Falco was my main back in SSBM, so I'd love to see him return, but I doubt that he will. For all intents and purposes, he was VERY similar to Fox. I'll keep my fingers crossed, but I'm not optimistic. (Doesn't matter, MK will replace him as my main regardless
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Ganondorf was a very popular character, and he is very well known. I think he'll make an appearance. But personally, I wasn't too wild about him being a Cptn. Falcon clone. The moveset didn't fit the Great King of Evil to well at all. I'd love to see him back, but with a brand new moveset.
Speaking of the Captain, I'm sure C. Falc will be coming back. He was too popular, and a fun character. I mean, c'mon, did you honestly ever get tired of "Show me your moves"?......Ok, maybe you did, but I still think he's returning.
My bro is a major Fire Emblem fan, he had mained Roy in Melee. He isn't to optimistic about the chances for Marth and Roy, as Ike seems to have taken both their places. He's fine with that, he likes Ike better anyways!
I think that Sheik will return as part of Zelda. Lots of people I know only used Sheik. Zelda is a good character on her own (the best Smasher I know personally uses Zelda, with very little Sheik, and he is DEADLY!), but I feel she needs the balance of Sheik. Even though she is in her Twilight Princess garb, I think Sheik will find his/her/its way into the game.
Pichu and Dr. Mario weren't very good clones in my opinion, and, as much as I love the LoZ series, I feel the same way about Young Link.
Jigglypuff.......I honestly don't know what I think. She was popular eough, and original, so I think there is a fair chance. But if any SSB64 character is getting cut, I think it's her.
Finally, though I was never a huge fan of him, I think Game and Watch will return as well. He has a rather enormous fanbase, too many would be disappointed to see him axed.
Sorry for such a long post guys!!!