So apparently this is the cool thing to do these days: Discuss Smash Bros 5. So I'll give my thoughts on predictions going around and maybe provide some of my own.
First off, there are so many people going around saying this game will be a new game, but I'm with the slight majority that understands that it will more than likely be a port, much like Mario Kart 8, Pokken Tournament, or Hyrule Warriors. My reasoning is that it was announce the same year it will be released (at least according to the current plan). Think of how long we had to wait on previous versions; those were full on new titles, and last time, the invitation came a full year after any gameplay footage was available. This time, it's this year, meaning it's already close to completion and there won't be too much beyond a few new characters (probably no more than five or six), stages, and at best a new game mode.
As for some of the characters themselves, I'll go over some of the most requested characters and give my thoughts on how likely they would be. Keep in mind, this is all educated speculation, and I acknowledge the possibility that I'll be wrong:
Waluigi: I get why he's been so widely requested, but I honestly doubt he has a chance. People cite Little Mac as an Assist Trophy that upgraded to full player status, but that's a pretty small sample size, and Little Mac had a game between installments. Waluigi has been in games, but he never had a game the same way Yoshi, Wario, and even Luigi and Peach have had. Hell, Rosalina and Bowser Jr have been treated better than Waluigi, leading to their inclusions. Waluigi has always been a filler character for side-series who never got a full outing, and since we already have plenty of characters paying tribute to the Mario side games, I don't see Waluigi happening.
Pauline: So Pauline received a glorious return in Super Mario Odyssey, one of the most successful games since Smash Bros 4. However, she was only relevant for a single chapter, and she didn't really do anything. In fact, I don't remember seeing her do anything in ANY game. No powers, no physical feats, no playable appearances, nothing to base a moveset off of. It would have to be entirely off the cuff, which is far too risky. I can see her getting prominent feature in a New Donk City provided that stage comes to fruition, but as a fighter? I seriously doubt it.
Geno: Every time I hear this name, I can't help but just release a sigh. We haven't seen anything Geno in years, and when we did, it was for a side-series game in which he wasn't the main character, and the game itself was a joint property, so on technicality, Geno is a third party character. This is a classic case of a character being heavily requested due to moveset potential. If they haven't done it already, they probably won't do it.
King K. Rool: See, this one actually feels pretty reasonable to me. I was never a fan of the Donkey Kong games (I actually find them rather boring), but I agree that the villains of the roster are lacking, and King K. Rool is well suited to fill the role. Multiple appearances, huge moveset potential, I mean why not? If they add him, cool. I'm not against it.
Ridley: This one I'm largely against. But what can I say that hasn't already been said? The problem isn't how big he is. There's plenty of characters that have been resized to fit into Smash Bros. But those characters can be resized without sacrificing what people love about that character. Bowser is customed to resizing for Karting and Tennis, Kirby and co. and Olimar are easy to forget that they're small because everything else is to scale, and the Pokemon were safely resized. Ridley doesn't have that. Bringing him down to scale would destroy what people love about Ridley as a ginormous boss. It would be like including Rayquaza in Smash Bros; it just doesn't work. I'd say Ridley's best hope is getting the Ganondorf treatment, by which I mean getting a more humanoid form as opposed to its imposing boss form, like Ganon. Ganondorf would never have been included otherwise.
Midna: I agree Midna is cool and has moveset potential, but I don't agree that Midna is Smash Bros material. She was relevant for Twilight Princess, whose time has passed already. They had two opportunities to include her already, and they opted not to. You can make Hyrule Warriors arguments too, but there were so many characters in that game, Midna being kind of minor in comparison, that it's not even worth bringing up.
Ghirahim: Everything I said about Midna applies to Ghirahim. Except that I like him less.
Impa: Impa would be an excellent choice and I would be totally on board with her inclusion. She's a badass with multiple games under her belt, and I loved playing her in Hyrule Warriors. Speaking of which, she had a pretty big role in that game, bigger than Midna and Ghirahim, so this can actually be used as an argument.
Linkle: Linkle is one of the characters I would love to see included above all others. She was my best character in Hyrule Warriors and plays completely differently from Link, so the risk of being a clone is minimal. The only problem I see is the nature of Hyrule Warriors itself. She might have a similar problem to Geno as she is a joint property, but you know what? Linkle technically has three games now, so screw you Geno.
Spring Man/Ribbon Girl: I don't like Arms. But will they include Arms characters? More than likely. I've heard arguments for more popular Arms characters, but let's be honest, they'll probably opt for the main characters instead. Probably as alternate costumes of each other with Spring Man being the base, similar to Robin, Corrin, and Wii Fit Trainer. Think about it, having a character with spring-loaded punches would be interesting enough on it's own, so does it really matter which one?
Rex & Pyra: I never really played Xenoblade Chronicles 2, seeing as I never finished the first, but apparently the main characters have cool abilities with a cool Rosalina & Luma gimmick, so I'd be okay with them being in. They sound like they would be fun characters. But I'm iffy about their odds. From what I understand, it took a lot of work to convince the devs to include Shulk in the first place, and they may opt to keep him as the sole Xenoblade rep if they even keep him at all.
Labo Man: So this has to be one of the weirdest requests I've heard so far, and you know what? I'm on board with it. Having another hardware-based character alongside my boy ROB would be pretty cool, even if I don't like Labo. What can I say? I see potential.
Decidueye: Of course I agree with this. You see my artwork? All I do is Pokemon, and Decidueye is definitely a favorite and prime candidate in my eyes. Don't listen to those Genwunners out there; picking Pokemon from the newest generation is the way to go. Hopefully I don't have to do too much work to convince you that he would have a badass moveset, either?
[insert Fire Emblem character here]: I've only recently got into the Fire Emblem franchise (through Fire Emblem Warriors, because I'm bored with the gameplay mechanics of the main series), and I can very easily understand why there are so many in Smash Bros already. It's a franchise on the rise with several dozen unique and lovable characters that were designed to fight. Plus, the success of Fire Emblem is partly credited to Smash Bros in the first place, so yeah, it has high billing in Smash Bros. As for a new character, I don't know where to start. They'll probably have no more than one, but I don't know which one. I've heard a lot of people talking about the character from the up and coming switch title, but I can't talk about them because they don't exist yet, so I'll focus on the ones I know about. Lyn would be a worthwhile upgrade from Assist Trophy, Celica got a recent reboot in Echoes and has a cool moveset, Tiki was a very interesting character from the earliest games, Anna is the only character to be in most of the games and has an interesting gimmick, and Rowan/Lianna are the protagonists of Warriors, so they would be awesome (barring the issues with Linkle applying to them). Any one of them would be a good fit.
Side note: I really hope they keep Lucina. I really like playing as Lucina, and she's a favorite character of mine.
Isaac (Golden Sun): This is another character that I don't know and can't really add to, but apparently Golden Sun was a popular game, and I'm not necessarily against it. I can only express concerns about how old the game is and how unlikely Sakurai is to come back to him.
Takamaru: I really don't know about this one. I mean, I guess he'd be cool, but I don't really like him all that much.
Simon Belmont: Ditto what I said about Takamaru.
Balloon Fighter: This one would be really cool. Imagine if Balloon Fighter had normal jumping capabilities without balloons, but with balloons he's better than Jigglypuff, and his balloons can be popped. His Up Special could involve inflating balloons (up to two), and a Down Special forces them to pop for a powerful attack. It would be a nice gimmicky character, and I think he has higher odds than most other classic characters. All they would have to do is give Villager a new recovery.
Ice Climbers: I never liked those guys. They're absence had no affect on me in the last game, and I would not weep if they continued to be absent. But they're gonna try to bring them back. I know they are. *sigh*
Snake: I loved him. He was my second best in Brawl; he was so fun to use and his Codec conversations were so much fun to listen to. If only Konami would stop being a bitch and let him come back. I'd like to see him in his MGS3 form.
Wolf: I liked him better than the other three, but I'm not sure if it was necessary to have him in Smash Bros. Star Fox isn't THAT big of a franchise, so two should cut it just fine. But Wolf was really cool, so bring him back if you can.
Pokemon Trainer: Oh my god that was such a cool mechanic. I can't believe they removed him just because they removed transformation. Bring that back. Doing so will also make it easier to stomach Sheik still being here despite being the least relevant character on the roster.
[insert Sonic character here]: No. They're not giving us another Sonic character. Guest characters are just guest characters and are not allowed a plus one, and until I see evidence of otherwise, that's how it will stay. Besides, out of all of them, Tails would probably be a priority, and other major characters like Amy or Knuckles would take precedence, but requesting Blaze or Silver is just stupid.
Banjo & Kazooie: I guess they would be alright. Again, I was never a fan, but it was a pretty iconic game, so he would probably fit in fine as a third party character.
Crash Bandicoot: He'd be pretty cool. I'd prefer him over Banjo & Kazooie.
Spyro: Having them both in would also be cool. Will it happen? No idea. Probably not, but feel free to hope.
Shovel Knight: I see the moveset potential, but would Smash Bros enter a third party character from an INDIE game? I mean, the ones that were included are either gaming icons or ballad winners. Same goes for Shantae, but I could be wrong. I guess I wouldn't mind.
Frisk: I'm putting Frisk here because, although Sans is the more requested character, Frisk is the main character. It's the same deal as Villager; they're not going to pick another character just because they're more popular. They're gonna opt for the main character, and give that character a moveset reflecting various assets of their game. That being said, although I love Undertale to death, I don't think it will happen. Undertale having a Switch port confirms nothing, and I still express the concerns of including indie game characters.
Goku: Why is this a thing?
Shrek: Why is THIS a thing?
I'm next going to go over some characters I'd like to see, but probably won't happen.
Zael (Last Story): The game didn't do that good from the start and it's time has since long passed. But I was so freaking excited to see his and Calista's trophy in Smash 4, and he was a top 20 for the ballad apparently. Also, he would have huge moveset potential. But I've given up already. He's not happening.
Zoroark: Zoroark is my favorite Pokemon. I was begging for him to be included in Smash 4. Unfortunately, Gen VI was happening, and Gen V was being forgotten because of all those people that hated Garbodor and Vanilluxe. I suppose there's a slim chance for Zoroark and his unique gimmick, but I also think his only chance was in Smash 4, and they opted for Greninja and left Zoroark in a Pokeball.
Maxwell (Scribblenauts): Scribblenauts is a fantastic franchise that fits right in with the Nintendo lineup, and it's a pretty active franchise. For those that don't know, Maxwell is this kid with a magic notebook that can spawn anything you write down (with the exception of inappropriate terms or proper nouns). One game even allowed you to spawn Mario and Zelda characters, so you have the connections to consider. The main concern is that Scribblenauts is not a Nintendo property; it belongs to WB.
Zygarde: I love Zygarde and I can think of many ways to turn him into a unique fighter. But I'm the only one thinking that. Zygarde never got his own outing and Gen VI is over, although he got new forms in Sun and Moon. So did Greninja though. They'll probably work with that instead (god I hope not, though. I hate the idea of Ash Greninja because I hate Ash).
Cappy: Here's an idea. What if instead of completely reworking Mario's moveset that we've been running with for years, we just make Cappy his own character? He could use captures that were featured in Odyssey, like Sherm, Gushin, Uproot, and Pokio, as well as standard attacks being cap throws. He'd be a much better choice than Pauline, but again, I'm the only one who thinks so.