My general predictions:
Anyone who was in the first Smash game is guaranteed a spot.
Any non-clone characters who have an upcoming Wii U/3DS game is a lock. Diddy is in. Sonic is in. Snake is out.
Clone characters are out.
Olimar will be replaced by one of the new protagonists from Pikmin 3.
Character Predictions (based on a combination of whether they had/have a game coming and my “gut feeling”):
Little Mac
His last game was Punch Out Wii, so he’s got the best shot out of everyone in my list. I’ve seen some people say that he’s a lock because of that ring stage, but I don’t think that’s a Punch Out stage specifically since the Smash logo is plastered on the mat.
Lloyd Irving
If any Namco character’s going to be in Smash, it’s this guy. He fits the requirements to be in a Smash game, he has a workable moveset, and I’m pretty sure Namco will have a character be immortalized in a Smash game since they’re working on it. Plus, the Tales of Symphonia pack is coming (though not for Wii U, ironically). Smash games can be useful marketing tools.
Mike Jones
This one is a long shot, but Smash games are known to have unexpected inclusions. We all know that Star Tropics is one of the most dormant series that Nintendo has. Making a moveset based on a yo-yo should be easy to do, not to mention the other weapons available to him. And let’s not forget this great fake image:
Spoiler
http://img357.imageshack.us/img357/6290/mikeap2.jpg
Fawful
There hasn’t been a Mario & Luigi (RPG series) character in a Smash game yet, so this guy would be perfect. There isn’t anyone else as memorable as him, and there’s no doubt he will be in the upcoming M&L game.
As for stages, I would like to see a Pushmo stage. Just take a look at the game and try to tell me you can’t see how its levels could work as a stage.