Brawl Mario seems to be a merger of Doctor Mario and Mario from Melee in stats. His ariel punch now hits characters down as well as KOs upwards depending on when and where the oppenent is hit. So the Doctor is gone.
Pichu is a joke. He is weaker then Pikachu is almost every way. I say he goes.
Roy was an advertisement, newer FE games are coming out. I say Roy goes but may be replaced by another FE character.
Young Link could return as Windwaker Link, but I think only one Link will return (although Wolf Link may make an appearance somehow).
Falco... I think some of his stats will go to Fox and then Fox will be nerfed slightly. Falco is too similar to Fox anyways, and with Krystal and Wolf high in the popularity polls I think one of them will join the canine instead.
Ness may be replaced with Lucas. Sakurai had a hand in Mother 3 and I heard that Lucas has many of Ness's moves. If he does replace Ness, it will probably be more of a physical change then a dramatic overhaul.
G&W doesn't seem like he will make it. I love the character, but his gimmick was suprise. It wouldn't be that suprising if he returned with the same moves or slightly buffed. Wario seems to be filling in the class clown roll anyhow. Don't worry I am sure Brawl will offer enough fun suprises to make up for his cut, if it happens.
There are rumors floating around that Jiggs was not going to make it into Melee. So is she on the chopping block? If she returns that means there will be three ball characters. I hope she stays, but it is an interesting question. I am going to say that Sakurai keeps her.
With my proposed 7 cuts there would be 19 returning characters. Plus the 5 new ones in the Brawl trailer. So 24 characters so far if my theories hold up. Maybe there only will be 30 to 35 characters. Probably something not dividable by 5, like 33.