I'd say anyone currently recognizable to the public has a fair shot of showing up in a Direct. That's characters such as Palutena, Dixie Kong, Chrom, Robin, Mii, Pac-Man, Ghirahim and if he counts as a newcomer, Mewtwo (for the record, these are not all characters that I think will get in). As a third-party character, Snake would probably also get revealed in a Direct if he were to return.
Anyone not straight-away recognizable to a wider audience is probably more likely to end up as a secret character, in which case I don't think they would be shown before release. that's not to say all secret characters will be obscure or unknown - there really isn't that much time between now and release to show lots of new characters, and some returning veterans are also gonna end up secret. The reveal of Takamaru, Isaac, Sable, Captain Rainbow, Lip, an Advance Wars or Rhythm Heaven character (again, not all characters that I think are gonna make it in) would mostly serve to excite their individual support bases, so I think they would leave most of those to be surprises for when the game comes out.
Characters like Ridley, Shulk and Tingle I consider to be borderline between these two categories. It's been a long time since Other M, and his Other M design is pretty specific to that game. Casual players of the older Metroid games might not even recognize his new look, at least until his name is shown. It hasn't been quite as long since Xenoblade, and with a spiritual sequel on the way it might seem like the obvious choice to tie in his announcement with updates on that game. X doesn't seem to feature Shulk, however, so (for now at least) the only people really aware of who Shulk is are the ones who played or have heard of Xenoblade. Tingle didn't make any appearances in Zelda games during the seventh generation, and his RPGs only came out in Japan (and Europe in the case of the first one). He's finally made his HD debut in the Wind Waker remake, so perhaps enough people are aware of him.