Assist Trophy: Birdo

Birdo is certainly an important character in the Mario series. She was the primary boss in Super Mario Bros. 2, along with appearing in dozens of Super Mario Party, Mario Sports, and Mario Kart games. I would say that she's more deserving than Waluigi for becoming a character, but she's another character that still isn't quite there. Maybe if they remake Super Mario Bros. 2 or if she gets a starring role in an upcoming Mario game, then she would be far more deserving of a slot.

For her move as an assist trophy, I think she could shoot eggs like she did in Super Mario Bros. 2.

Stage: The Impact Site

I personally thought that the "Distant Planet" was a really stupid concept for a Pikmin stage. Why couldn't they have named it after one of the stages? I think that the Impact Site could be a very similar stage to the distant planet, with a few exceptions. One thing that could change would be that there would be something instead of a bulborb at the right. Maybe a wall that could be broken with enough damage or another piece of scenery. The ship in the background would still remain there.

Stage: Saffron City

We need this stage back for archive purposes. Saffron City was pretty simple, but was a really enjoyable stage.

Trophy and Sticker: Seahorse

Who can forget the seahorses in Majora's Mask? They were so cool, but I can't think of any way to be an important role in Smash Bros.

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.

Item: Teddy Bear

The teddy bear was an item in Earthbound you could buy. It would follow you around and take certain damage for you from enemies for around 200 health points. I think it would be an awesome addition as an item, because it's a unique concept.

Assist Trophy: Ivan

 

Ivan should be the final of the three assist trophies to appear in Smash Bros. 4. He could use his Earth PSI or his staff to attack enemies.

Trophy and Sticker: Balloon Fight Enemy

The only enemy in Balloon Fight that you can defeat. It is important enough in my opinion to be both a trophy and a sticker.

Sticker: Marcus McCloud

The son of the greatest mammal in all of Corneria, Marcus has never been seen in action, but should get a little bit of recognition. I'd say a sticker would do the trick.

Pikmin Enemies

 Yellow Wollywogs could jump up like they do in the Pikmin series, then slam anything below it

 Firey Blowhogs could spit fire at you

 Puffy Blowhogs could not do damage, but could push you or blow you from advancing

 

Bulborbs could be common enemies that bite you  rather than eat you

 Goolixes could move around, but you'd have to hit it's weak spot. If you hit its other spot, it would send you flying

 Mamutas could slam you into the ground

 

Armored Beetles could be a mini-boss or at least a very tough enemy

 

Pearly Clamp Clamps could hold items like a pokeball or a sticker, but could swallow you for a whole bunch of damage

 

Swooping Snitchbugs could grab you and throw you to the ground

Captain Olimar

 First Appearance

Pikmin

Gamecube

2001

The Captain will be another obvious choice. Only problem for him is whether or not his own series will continue. Nintendo has been very reluctant over the past decade to release anything out of their "circle" of games, which would be Zelda, Mario, Kirby, Pokemon, and Metroid. This mentality is stopping a lot of great series in its tracks like F-Zero, Earthbound, Donkey Kong, Starfox, and of course Pikmin. Nintendo has been saying every year since 2008 that they would release a new Pikmin, but they have yet to do so. Unless this game is a perfect game, there will be a lot of upset fans, more than there already are.

I would like to somehow see mushroom pikmin and bulbmin to somehow appear in Olimar's arsenal. Other than that, unless there's new Pikmin species in the upcoming Pikmin game, I think there's no need to change Olimar's moveset in any way.