Okay, so here’s how my roster prediction currently looks:
The usual OCD stuff:
Wario’s next to Pac-Man (they both like to eat)
Falcon is above Olimar (“Captains”

Falcon is next to the Star Fox characters (shared universe)
Olimar is next to the Kirby characters (tiny characters)
Olimar is next to Villager (2001 Nintendo EAD creations)
R.O.B. is next to Mega Man (robots)
Snake is above Sheriff (weaponry!)
All the RPG characters are together
All the Disk System characters are together
The new guests are diagonal from each other and so are Brawl’s guests
Some comments…
Dixie Kong
I wouldn’t be surprised to see Dixie in the game, but I also wouldn’t be surprised to see her excluded. I really think her inclusion depends on whether or not Sakurai knew she’d be in Tropical Freeze when he was putting the roster together. Otherwise, it’s tough to justify including her without that push.
Sheriff
Really, this could be any bizarre historical character, but I went with Sheriff for the time being so I could attach a name to the box. I definitely don’t think he’s set in stone.
Zoroark
I’m still one of the crazy few who believe Zoroark has a good shot at being included. The arrival of X/Y doesn’t mean Black/White/Black 2/White 2 don’t exist, and the roster needs more post-Brawl characters. The Dark type is unrepresented within the roster of playable Pokémon characters, so there’s a lot of room for a unique play style. I’m sure Sakurai could do something with the Illusion ability as well. I’m not behind Zoroark 100%, but I do think it’s more likely than most people want to believe.
Characters I could see being included but didn’t put on my roster:
Toon Zelda
Zelda is big enough to deserve another character, and I’m absolutely convinced Toon Zelda would be next on deck. Given that Sakurai has repeatedly stated how important it is that newcomers offer something unique, I wouldn’t be surprised to see a Zelda clone excluded. I also don’t know if Sakurai would want to include a third “magical princess”-type newcomer after Rosalina and Palutena. That might be overkill.
Chrom
Chrom’s popular, but he’s also really boring. Between Marth and Ike, Chrom just doesn’t seem
necessary. A Chrom & Lucina pair character would remedy that, but I’m not sure how technically feasible a character like that would be since Sakurai couldn’t get Diddy & Dixie to work together in Brawl.