Make your voice heard
Midna
Midna may already be an assist trophy, but with skills like hers, it'd be a crime not to make her a fighter. She has the ability to teleport and create portals through the Twilight world, which could be used together to make incredible combos. Her hair would come in handy for her standard attacks, and for her final smash, she can turn into a giant gold spider and rampage across the stage. No one would be safe from her many-legged majesty.
Phoenix Wright
He might be a lawyer, but Phoenix Wright's actually no stranger to a good, old fashioned street brawl, thanks to his appearance in Ultimate Marvel vs. Capcom 3. Slap your opponents with a well-timed piece of evidence, or use the I'm-clearly-bullshiting-here hand-behind-head-with-sweatdrop pose to deflect attacks back at the opponent. And his smash attack? A shout of 'Objection!' and a finger-point that will send the accused flying across the stage.
Professor Layton
While this English gentleman is most famous for using his wits to solve countless puzzles, Layton's also an adept adventurer. He's just as comfortable scaling a mountain or fencing as he is at thoughtfully drinking tea. The heat of battle doesn't give him much time to decipher logical inconsistencies, but Hershel Layton is mentally (and physically) nimble enough to hold his own on Final Destination. Hell, Layton will probably be able to keep his hat on straight the whole fight.
Splatoon's Inkling
Splatoon's humanoid squids deserve to hop on the Smash roster. Their ink-shooting skills would make them masters at ranged attacks, though melee is a little harder to imagine - maybe whipping people with their tentacle hair? Meanwhile, the Inklings could zip around stages thanks to their ability to swim through solid objects. If they get in trouble, they can blast away from danger via cannonfire. They'll already have their own amiibo by year's end, so they seem like a no-brainer inclusion.
Birdo
Birdo seems to live in everyone else's shadow, as the dolled-up dinosaur mostly finds herself relegated to spin-off status these days. But that doesn't change the fact that she's a fierce competitor, and would be right at home in Smash Bros. Heck, she could bring out all the sporting equipment she's accumulated over the years and finally smack the everloving crap out of Mario and friends. Oh, and she'll have her trademark snout-based egg shot, of course. That goes without saying.
Simon Belmont
Like Mega Man, Simon's well-suited for the Smash Bros. attack system. All of his special moves could be sub-weapons from the Castlevania games; Simon might throw an axe in an upward arc, or toss a boomerang-like cross, or coat the ground in a puddle of holy water. And nailing a Smash Attack using Simon's whip - complete with a loud cracking noise if you make contact - would be incredible. Of course, you'd need to have alternate costume colors that represent everyone in the Belmont bloodline.
Andy
This is the kind of character that shows how Super Smash Bros.' esoteric roster can really shine. Andy (or heck, one of Advance Wars' many COs with some creative alternate costumes) would use a variety of different units in his arsenal. One move could let him hop in a slow-moving tank, another could send a small group of adorable infantry to charge out in front of him, and his Up-B attack could send him skyward in a helicopter. It's a bit outside-the-box, but it could totally work.
Lark
There's already a Pilotwings stage in the game, but none of the currently included characters feel like they really fit there. So why not add a super obscure, but no less beloved character like Lark who could call this stage home? Fans of Nintendo Power would recognize him as the comic character Nester, which could boost sales through nostalgia alone. And Lark could play like Metaknight and his ridiculous air game, gliding around and zipping through the sky with his jetpack.
The Ice Climbers
Fan favorites like Mewtwo and Lucas can return as DLC, so that should open the door for Popo and Nana, right? Director Masahiro Sakurai cut the characters because the duo's movements were too taxing for the 3DS's hardware, but what about the more powerful New 3DS? True, putting the violent Inuits only on New 3DS and Wii U limits who could play as Ice Climbers on portables, but the return of this fan-favorite fighting team would be worth it, no matter the playerbase-splitting cost.
