Stage: Podunk

Basically Podunk should solely appear as Ninten's home stage, considering it's his home town.

Item: Cucco

The most iconic animal in the Zelda universe, cuccos are annoying and useful and the same time. You could use a cucco to float for a long period of time. But the downside to these birds would attack you in large numbers (just like in the games) if you hit them with enough damage.

Assist Trophy: Matthew

 Matthew is the son of Issac and because of this along with his important role in Golden Sun, he should appear as an assist trophy.

Mother/Earthbound Enemies

 Clay Men would be tough enemies to beat, but would be very slow.

Fobbies and Foppies are perfect just for punching bags. Also, they're very popular to Mother fans.

Various Pigmasks would be an obvious choice, just to bash them around until they squeal.

 

 Giving some Bosses some love as mini-bosses would be cool, too.

 Starmen and its variations would be perfect. They're probably the most popular enemy in the entire Mother franchise.

 Mother has perhaps the craziest, fun, and wildest enemies in video games to date. I think that if Nintendo can't release the games in the US due to copyright, they should at least give a little extra content in Smash Bros. After all, it's been 16 years since we last saw a Mother game in the US.