Every character from the first SSB is in both Melee and Brawl, and there are only 5 Melee characters who aren't in Brawl. So it wouldn't be too hard to add them back in for the next one.
Not at all ... >_>
Roy was added for promotional purposes. He was the advertisement for the EPIC FIRE EMBLEM RETURN (exclamation marks) on GBA. At first, he was even supposed to have Sigurd in his stead. >_>
Ike represents the newer Fire Emblem games, and replaces (the old) Roy extremely well, if you place yourself with Nintendo's point of view.
Sakurai claimed that Dr. Mario had been added for the fun, and because of his Music Theme.
Young Link is replaced by a much more visually recognizable character, and that occupies the exact same role. Moving on ...
Mewtwo's departure was a surprise to me and alot of people, and I found kind of a shame that they didn't put the Pichu Bros to help Pichu be more original.
These two are the only ones who can have a bit of a chance, though if no new significant official material is produced about them between SSBB and SSB4, they hardly have any chance. They'd be pretty much
too old.Melee was Melee. It was launched at a completely different time, in a completely different context. Stages, characters, music, even some items, are chosen according to their level of importance and popularity at the moment a game is launched. But ... when was the last time we saw Mewtwo have a decent role somewhere ...?