I don't think we can predict who exactly will be announced, but I made some thinking and I think these are the general lines Sakurai should/will follow:
- One character from a new, fresh franchise. Most likely Shulk, but they could go all the way and put the Wonderfuls in it.
- One character very requested, and in these I put Little Mac, Ridley, Mewtwo and King K. Rool.
- One character from a well known franchise already represented in Sm4sh, and possibly not very expected. In these I include Paper Mario for example, or some pokémon like Genesect. Or maybe Bowser Jr., or Impa/Ghirahim/whoever new character Zelda will have.
- One character showing a complete revamped moveset. This could be Luigi in his Dark Moon outfit, or Ganondorf with finally the moveset he deserves. Possibly Pit using some of the weapons in Uprising.
- One Namco/Bandai character. I didn't say "third party" because I think that Namco is obviously in a special position this time around. Technically is still third party obviously, but though... Pac-Man is my bet (a new Mario Kart GP gets released in Japan this summer), but pretty much anything could happen.
- One third party character again. Capcom's Megaman, Level-5's Layton... Endless possibilities.
Also an unexpected character falling into these categories:
- One completely unexpected obscure retro character and I mean really unexpected. "People don't know who he is" level unexpected. This is hard to pull off but I trust Sakurai. Hanafuda would be a perfect example.
- One character considered almost by everyone impossible. Geno, Banjo-Kazooie, something like that.
- One original character, alternatively, which will be the main villain of the new story mode.
This is assuming the trailer will show a bunch of characters, I'm expecting 5 or 6. Pretty much everybody is expecting 5 so it would be fun from Sakurai to add a sixth character post-trailer, like Snake in the Brawl one. Especially if it's the obscure one.
My final picks are: Shulk, Little Mac, Paper Mario, Luigi with the new moveset, Pac-Man and Professor Layton. For the unexpected, I can't really say, but I think someone will actually show up.
Also I would like the trailer to have some hints for the rampaging speculations which will flow after the reveal.
Something like a tiny blue things running in the background hinting at Sonic, for example.