I see a few possibilities. Here’s a list of who I think might be likely newcomers and veterans:

Newcomers

Miis - Unlike most people in the Smash community it seems. I don’t hate the very idea of Miis being playable. They have their place in Nintendo now. And have a good enough resume to be playable. Ever since Wii Fit Trainer was announced + the THREE Mii related stages we have. I think they’re shoe-ins. A Smash Bros. related direct would be the perfect place to explain how they’ll work.

Ridley - Maybe it’s just because Monday’s screenshot happened to have a Metroid. But there’s also this feeling a really hyping reveal will come out of this. And Ridley’s pretty much the most requested Nintendo newcomer.

Palutena - There’s a supposed leak out there that hasn’t exactly been 100% debunked yet where it looks like Palutena is playable. And it might be time we see if it’s true or not that she’ll be a Newcomer.

Honorable Mentions: Pac-Man (I think if Pac-Man’s playable he’ll be an E3 reveal. But maybe Namco had other plans), Shulk (For Xenoblade), and a Fire Emblem newcomer

Veterans

Yoshi - Alot of people thought he’d show up when Yoshi’s New Island was released in the US. But maybe the reason we had no character reveals in March was so we could have multiple during the April direct to hype us in one sitting. Yoshi’s a guaranteed returnee either way though. He’s not one to worry about if he doesn’t show up once again.

Snake - Snake has been a bit iffy on his chances. And if he’s in the game, he’s likely going to have a big day for himself just like Sonic did last October. So this may be the day to put the doubts about Snake returning to rest. Because unfortunately time’s running out for the poor guy. If we don’t know about him being in the game before Smash 4 releases. That’s probably not a good sign.

Wario - After Yoshi wasn’t revealed, many people pointed out that WarioWare had an anniversery in March. So he might be March’s character reveal. Needless to say, that didn’t happen either. But it might be the fat man’s return come tomorrow. Although Wario is not one of the Original 12 nor was he in Melee though, Wario is a shoe-in to return just like Yoshi. You can’t have Ashley from WarioWare as an Assist Trophy if Wario isn’t playable.

Any Popular Veteran from Melee and/or Smash 64 that hasn’t been revealed yet (This includes Ness, Captain Falcon, Ganondorf, and Mewtwo) - It’d be pretty cool for a character like Captain Falcon to show up since he’s beloved because of the Smash Bros. series. Mewtwo might also be a good one since he missed out on Brawl. And is probably the most wanted Veteran.

Of course knowing Sakurai I could be completely wrong and he surprises me by not showing any of what I think might come. But that’s the fun of it all. ^^