Stage: Hyrule Castle

I think Hyrule Castle should reappear as it did in the N64 version. In other words, it should not be updated graphically. Smash Bros. is known for having different styles of graphics in the game, why not add a vintage Nintendo 64 style?

Item: Mushrooms

 We all know the two types mushrooms in Super Mario Bros. However, in the Galaxy series along with the New Super Mario Bros., there were many new types of mushrooms. I think we should see a few of these in the next Smash Bros. The mega mushroom could make you as big as possible for a little while, but is much rarer than other mushrooms. The bee mushroom could give you the ability to fly briefly without any limitations. Lastly, the stone mushroom could make you unable to be knocked back, but in exchange you'd be slightly slower.

Assist Trophy: Mia

Mia could use her water PSI as an assist trophy. She could summon a wave with her staff.

Assist Trophy: Toad

I think before Toad becomes a character, he needs to be an assist trophy. He could just jump up and down saying "(insert character here) the princess is in another castle" and used as a distraction.

Assist Trophy: Eliwood

 Eliwood would make a great assist trophy. As Roy's father and protagonist of the first American Fire Emblem game, he definitely qualifies too.

Assist Trophy: Myyrh

 Myrrh would be an awesome assist trophy, primarily because of what she can do. To see this cute little girl transform into a dragon and blast your opponents away would be priceless.

Assist Trophy: Miciah

Very deserving of being a character, but there's simply characters who deserve the slot more than her. I can see her very likely appearing as an assist trophy because of her importance to the series. Maybe she could call her bird to attack you, or use magic.

Assist Trophy: Adam Malkovich

Adam Malkovich from Metroid the Other M should be an assist trophy for sure. I would say he would make a good character, but I don't think 1 game as a supporting character at this point cuts it. I don't even know if much wider known characters such as Waluigi should make the cut, so Adam is out of the question at this time for me. Adam would be a great addition to the assist trophy collection, though. He could go out and shoot at you similar to Saki from Brawl.

I think if Adam continues to play an important role in the Metroid series, he would be a great addition to Smash Bros., especially with a major series so low on character options.

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.