My opinion on who will be cut:
Sonic- He's a third party, viable for ONE game. They'll want to make room for other third parties, unless they give in AGAIN to the massive fanservice. WE ALREADY KNOW WHO WOULD WIN MARIO VS. SONIC PEOPLE: MARIO.
Snake- Same reasons as Sonic. Plus, I'm pretty sure Hideo Kojima is tired of giving Sakurai blowjobs for making Snake too good.
R.O.B- I don't see him returning, but then again, there IS the MGNW rule... Although R.O.B. isn't the creation of Sakurai's mentor, so he might get cut...
Pokemon Trainer- One way or another, he'll either be replaced by a more recent Trainer or another Pokemon.
Marth- Really, this is a coin flip. He's the flagship of the Fire Emblem series, yet he's as old as his first game. He MIGHT get replaced, but then again, flagship characters are important. Just my opinion.
Pit- He's fanservice too, although at a much lesser scale. He probably won't be returning.
Toon Link- Clone. To be SURELY replaced.
ZSS- Fanservice. To be ditched altogether.
Lucario- Scrapped for Mewtwo's Revenge. He'll likely bring along Darkrai and another dude too.
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And now my opinion on the popular demands!
GENO- Absolutely not. Not a single mention of SMRPG in ANY of the Smash games, and he's only important for storyline purposes and as a partner. People who claim he has a more important part is like saying Mallow or Bowser or Peach does. Really, only one has the MOST important role- Mario. Unique moveset? So he can transform his arm into other weapons. We already have a more important third party with that ability- MEGAMAN. DENIED.
BOWSER JR- THIS here is a character; a major villain in THREE mario games, and has appeared more frequently as time passes. His brush and magical abilities can give way to a fun and unique moveset exclusive only to him, and his small stature and power could give way for an interesting character stat-placing. They better not give him Shadow Mario, though. SM= Mario with recolored Beam Sword. ACCEPTED.
PAPER MARIO- A good side character to Bowser Jr, the Paper Mario series has become widely successful and ALL of his games have been the best on their individual systems. A 2-D character, MGNW, may already exist, but this shouldn't stop Paper Mario from gaining a spot, with his undoubtably unique moveset and animations. ACCEPTED.
KING K. ROOL- He would've been great in Brawl, and probably WOULD have made it, if Diddy was in Melee. A recurring character in a highly successful franchise deserves to be recognized, except in his King costume, not the pirate or mad scientist. Plus, we need more mentally insane heavy characters. Wario should NOT be alone. ACCEPTED.
VAATI- A bit of a coin flip if you ask me, with a slight tilt towards a denial. He's been only in a handful of Zelda games, all of them featuring Toon Link who will likely be ditched. Why add a villain to a roster where the hero is gone? DENIED.
WOLF LINK & MIDNA- I'd have much rather preferred to see this guy in rather than Clone Link. They couldn't be ABLE to clone this dude/dudette. Plus, we don't have many quadripedal characters, so it should make for an interesting character moveset and design. But could he make it into the next one? Probably not, but could he have made it into Brawl? YES. YES. ACCEPTED.
DARK SAMUS- I find this character absolutely unoriginal, save her birth. That's a pretty cool cutscene. Other than that, why the hell would this CLONE get in? Important villain? Ridley has that. Cool moveset? Ridley AND Sylux have that! Ditch the evil clone and stick with the dudes. DENIED.
RIDLEY- Now we're talking. Too big? Bowser is the size of a school bus in the Mario games and Olimar is roughly the size of a quarter. That's pretty **** big AND pretty **** small. So that's moot. Furthermore, he's been getting a "Promotion" over time. Intro baddie in Melee to Brawl boss fights. Sooner or later, he'll have his day. ACCEPTED.
KRYSTAL- I used to like her. She's female, wields a staff, and makes for a good character. Then I realized what they did to Falco. And I realized I was wrong on every account. Her most important role in a game isn't even a proper Star Fox game, Falco goes to show they'll clone whoever they can without remorse, and she hasn't used her staff since Adventures, which even then, Fox McCloud now owns. I'll take Leon Palowski any day, yo. DENIED.
DEOXYS- An unsung hero. Of course, he probably has 0% now. But he would've made Brawl plenty more awesome. His Form Changing, tentacle ****** days could be over. But in Brawl, he remained viable. An incredibly unique character in the game, if not in his own series, should ALWAYS get a spot. ACCEPTED.
MEWTWO- Where'd he go? He's in line for Lucario's replacement. Do I really even NEED a reason? ACCEPTED.
DARKRAI- Another unseen dude, he's gained tremendous popularity in Japan; as did a certain singing puffball we all know and love/hate. Also, by Sakurai's standards, Darkrai had as much of a chance as Luca did of getting in. I guess Sakurai likes pants wearing Pokemon more. R.I.P. unique awesome character with a fanbase the size of Africa. ACCEPTED.
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Just my two cents.