Well, I'd say, considering the aformentioned capacity of the Wii, "all" of the 25 melee characters may actually return.
However they may just as easily not.
The original 12 are 99.9% guaranteed to be in.
Bowser, Peach, Zelda and Ganondorf, based on their popularity, are also an extremely high guarantee
Dr. Mario: Well, if they're planning on leaving characters out, his case is a bit of a 50/50 case. My reasoning for this: it's likely that there'll be clone characters in SSBB as well. Considering Mario is the most popular Nintendo character of all time, it's near guaranteed he'll have a clone character. Dr. Mario makes a perfect clone, then again, he might be replaced with another of Mario's countless versions. (Racoon Mario, Wing Cap Mario, Paper Mario, Baby Mario, etc.)
Falco: 50/50... He's popular yes, but not as popular as some of the aforementioned.
Ice Climbers: Well, not as popular as some may think you know... Pit is more popular and he wasn't included in Melee. If one of the starters is going to be thrown out, they're the most likely.
Pichu & Young Link: In my opinion, the most likely clones to be thrown out, especially Young Link. Twilight Princess if I'm correct doesn't have a Young Link and Link in SSBB is based on Twilight Princess. And Pichu, well, out of the clone characters, Ganondorf, Falco and Dr. Mario simply are more popular...
Marth & Roy: Well, after seeing the Fire Emblem stage I'm pretty confident that least Marth will return. Roy was merely his clone yes, but in SSBB amazingly popular because of his, well, cheapness. Marth more likely to stay than Roy but my guess is they'll both stay (and probably be joined by a char from FE9)
Mewtwo: Probably the most popular "evil" pokemon ever. 75% chance at least that he'll return.
Mr. G&W: Heh, I find it odd that so many people think it's likely for him to be left out... He's the oldes Nintendo character ever, has one of the most original movesets ever and because of his major nostalgic value makes, just as in SSBM, the perfect "final unlockable."
90% likely he'll stay, though, you never know.
That's my thoughts on it.