I'm posting this information to see if any of this actually comes true.  Obviously take this with a huge grain of salt because I don't know if any of this is true or if this guy is a big liar.  In the Bay Area the smash scene is pretty big.  In SF, they hold smash gatherings every week (In Oakland as well "the Made").  Tonight there was a guy who says his internship with Nintendo had just ended.  Yes, Nintendo has an office of some sort in Redwood/San Mateo (Bay Area).  Anyways, I'll just share the basics to keep it simple. 

The following chars are supposedly to be revealed

Bowser Jr. and The dog from duck hunt

Not returning/added:

Wolf and Mewtwo

*49 chars total

multiple skins available such as Dr. Mario. and Jr. Bowser's alt. costumes are the koopa kids. 

I'm not standing behind this guy by any means, but we'll see if any of this comes true.  

The dog from duck hunt? Im going to say fake on this one.

No way a intern has much access to exclusives. Plus these just sound terrible. There is no way.

The chars are not what I'm skeptical about (wii fit trainer), it's the entry level employee that supposedly has this information.  I agree 100% with you about him having this information and its the same thing I told my friend. "There's no way he has access to that information."  However, I don't put it past Sakurai to put these chars in the game. 

Bowser Jr. is not a koopaling, debunked.

All the chars are thematic in the NSMB games though.

I asked my uncle who works for Nintendo. He said it wasn't true.

I could believe Bowser Jr, but the dog from Duck Hunt as a playable character?

I guess Kin Corn Karn and the mobster from Hogan's Alley are next...

yes, seems logical, but making all the different kooplings as skins seems like too much work.