I mentioned this in another thread but I see a pattern in how Sakurai and company "cut" characters. And by cut, I mean remove characters that were present Melee that were not present in Brawl. If we look at the cuts that were made (Roy, Pichu, Dr. Mario, Young Link, and Mewtwo), four of them were clones and one of them received a successor in terms of a similar moveset. Young Link also had a successor in Toon Link, but Toon Link is still a clone of Link regardless. Mewtwo replaced by his game's next generation, Lucario. So under this pattern, the first individuals to be cut (not make it Smash 4) would be clones, and characters whose moveset could be moved into another generation. (Note: for the sake of argument, I define clone as anyone who shares similar special moves).
For the clones, the possible clones are Falco, Lucas, Wolf, Toon Link, and maybe Gannondorf. I would resist putting Gannondorf anywhere near this list since he's staying; it's just a matter if they continue declone, make him Ganon perhaps, or keep as it. Falco seems to have staying power. Wolf is Fox's second clone. Lucas unfortunately is a character whose game hasn't been released worldwide and also is a clone. And Toon Link, well, he's kind of a continuation of Young Link anyway. So out of the first cut, it would be Wolf, Lucas, and maybe Toon Link. But I wouldn't be surprised if Toon Link was replaced with another Young Link from ALTTP and ALBW. Sakurai has mentioned that he believes in the importance of having a young Link and an adult Link.
For the generation cuts, the only characters whose series I could see as generational is Lucario, Pokemon Trainer, Ike, and maybe Olimar. For these characters, they would be cut, but a new character from their series would take up their moveset. Olimar seems so important to Miyamoto so I doubt they'll cut him. And Pikmin 3 hasn't even come out yet, so that might impact their ability to replace him with a new captain. Lucario's time as Pokemon of his generation is up it seems. They are other Pokemon receiving the limelight and judging on how Sakurai say they pick the next pokemon, I doubt he'd be the pokemon they would recommend. And the new Mewtwo just seemed so primed to replace him. For Pokemon Trainer, the concept is so good that I doubt we'll see it cut, but it can work with any generation of starters. Black and White were the first pokemon duo to receive direct sequels, so I wouldn't be surprised if that generation got the nod. Oshawott, Servine, and Emboar (my personal pick for the choices, it also follows PKMN trainer's formula) are my favorite starters since the original.
Lastly, Ike has been supplanted by the Awakening generation. As an Ike fan, I would hate to see him cut, but under this pattern he would also be cut and replaced by Chrom. Chrom doesn't fight exactly the same, but there is enough commonality for him take over the specials. Chrom's Critical has a similar jump to Aether; Chrom's aether actually looks like Ike's quick draw. Eruption was a made up move anyway but it would be easy to turn blue. It all depends if Fire Emblem gets a third slot. Personally, Chrom isn't even the Awakening rep I'm rooting for, that would be [spoilers] lucina, but considering I couldn't even say the character's name, there are definite barriers there.
So if I was a betting man (which I'm not), I would bet the second cuts would Lucario and Pokemon Trainer. I can't in good judgement say Ike, but my thought process is there. Additionally, I would not bet on any previously cut clones returning (a Link from ALTTP and Newtwo would not be a returning clone). The most requested, a Roy return, whose game wasn't even released worldwide, does not seem likely due to his place in Nintendo history and Fire Emblem history. Not having a worldwide release limits people truly knowing his story, and limits his appeal as having a place in Nintendo lore. If he were to return, he would either be a clone, or decloned, which would be similar to making a new character. There are other lords since FE6 (his game) who could also easily be clone of Ike, Marth, or the new Awakening Rep.