For the lazy people, all three commercials together:

We had seen the soccerball before.

Training Update

I always though it was from Mega Man Soccer

OMG, Mega Man Confirmed

The fact that it catches on fire makes my theory 100% true.

Okay here is what I know. Most of it isn't particularly surprising, and the 2 newcomers I know of don't interest me.

Veterans:

Captain Falcon
Ganondorf
Ness
Marth
Sheik (Zelda transformation of course)

Newcomers:

Wolf (this is entirely from that guy's GameFAQs post, see below, so I don't know if he's playable or an assist or what.)
Lucario (from 2 sources so I'm pretty sure he's in)

Stage:

Green Hill

Music:

Green Hill music is not confirmed, but there is quite a bit of Sonic music, specifically at least one from Sonic CD

Source 1:

A friend sent this:

http://i2.photobucket.com/albums/y39/chupelo/gamefaqs.jpg

That was too spot on to be a guess.

Source 2:

An online acquaintance who knows somebody at Nintendo. She predicted the ability to have multiple BGMs per stage before it was on the blog, and also claimed Lucario was in. She's the one who talked about the Sonic music, Green Hill, and Marth. I trust her more than the GameFAQs guy, of course.