I have some questions:
1. Are the Chorus Kids or Karate Joe distinct enough in personality and design to be added in Smash? They look like webcomic stick figures to me.
2. How important is Impa if her appearances as an old lady don't count? I really think they don't count because a character has to be cohesive, otherwise why make the series about characters? Add a ****ing Goomba then.
3. Can someone just help me with my roster already? I'm designing it to have all the remaining All Stars, but I'm missing three characters. So far (in addition to Ridley, Inkling, Daisy, and Snake) I have Bandana Dee, King K. Rool, Dixie Kong, Paper Mario, Celica, Isaac, Rex & Pyra, and Crash Bandicoot.
1) Well Karate Joe has been in every single Rhythm Heaven game, and you'd likely be able to gather a personality of the character from his appearances across each game. Their 'webcomic stick figure' design as you put it would actually allow them to stand out more.
2) I don't see why her appearances as an old lady wouldn't count. We don't judge Ganondorf's importance based on just his human appearance, otherwise he'd have only starred in roughly... 3 or 4 games of the Zelda series.
Each of the Impa's, much like the Link's and Zelda's have been different anyway, even if the Ocarina of Time, Skyward Sword and Hyrule Warriors were more athletic designs. Role wise, I feel she does a lot more in the latter two than OoT.
If we're talking important Zelda series characters though, after the trio of Link, Zelda and Ganondorf, technically the next most important character would be Tingle. He's the only Zelda character whose gone on to be the lead character in his own games; 3-4 of them I believe. You could even say he was once playable in a mainline Zelda game as well, through the usage of the Tingle Tuner in Wind Waker. So if we're rating Zelda series characters solely by importance, I'd say Tingle is more likely.
3) I can understand Badanna Dee, King K. Rool and Dixie Kong, but what makes you say the other 5 count as all-stars (well, Crash is an icon, but not for any Nintendo appearance). In terms of likely additions, I'd say Isabelle and Ashley (WarioWare) are likely additions from pre-existing series in SSB. I would also count any lead character of a Nintendo IP as a Nintendo All-Star, which there are plenty of series without playable reps. Oh, the Yoshi series and F-Zero series are the only two SSB64 series without 2nd reps... so might be worth suggesting Kamek and Black Shadow (not sure on their chances though).
In terms of new to SSB series additions... well Dillon from Dillon's Rolling Western could work wonders and has 3 3DS games (the latest is even physical in some regions). He appears to be a good rising star. For a retro series addition, I would say the most likely currently are Balloon Fighter, Exitebiker, and Lip, though I feel it worth mentioning Bubbles as well (from what I've gathered, she appears to be Nintendo's first female protagonist). I'd have mentioned Starfy, but he's already an Assist Trophy. Art Academy and Style Savvy might not directly appeal to the SSB crowd, but they are important Nintendo series, and could have a playable rep much like Wii Fit now does.
I'll admit, I'm only throwing out suggestions here rather than a concise answer. Who have you got on this roster of yours so far?
There honestly aren't a ton of new franchises to pull from at this point. Particularly from circa late 2015. As Souldin mentioned, we have a lot of the smaller budget Nintendo characters like Dillion and Qbby. None of those characters really have had much of an impact, and are generally not as supported by the community either. Dillion is the only one I've seen with really any support, and at least he was an assist trophy in Smash 4. I'm inclined to say he is possible, but his inclusion could go either way.
Captain Toad was pretty much the only new major IP around that time, and he was one of the few breakout new characters from the Wii U/3DS era. So I'll say he is possible as well. ARMS is so recent and the comments from the producer of that game don't inspire any confidence in an ARMS character's inclusion. ARMS is the only one I see, Ever Oasis and Sushi Striker especially are entirely too recent entries and probably lack the fanfare or initial moveset viability to be included.
Everything else from that period comes from established franchises. We still do have lots of old characters to pull from. Golden Sun, Chibi-Robo, and Rhythm Heaven are all more recent IPs to pull from if Sakurai wanted to include them, with Isaac having the benefit of being a highly requested character and Rhythm Heaven having support from Smash 4's supposed cuts.
That basically leaves further back at that point, so retro reps or more one off characters. Geno is in a bit of weird place, but kind of represents Super Mario RPG more than just a Mario newcomer. Lip, Mach Rider, and Excite Biker are some other retro characters from unrepresented series/games.
There's just not a ton of new franchises Sakurai has to pull from at this point. There are a couple more glaring omissions in modern IPs, and DLC makes ARMS fair game I believe, but mostly the roster could use spreading out representation to other beloved series. For newcomers, that's basically the choice that will satisfy the most people. We're at a wonderful point where there are limited options for newcomers. We have increasingly obscure newcomers from old games as one choice, third parties as another, bolstering represented franchises with new characters, or filling the lineup with Pokemon and Fire Emblem characters
I'd argue that there are plenty of series Sakurai could pull from, Nintendo posses an unbelievable amount of IPs, it's just questioning which ones are more likely/important to add in first. The Wars series stretches back to the Famicom/NES and has many games under it's belt thanks to a revival on the GBA... but the future of the series is uncertain. Sin and Punishment got itself a sequel back on the Wii, but might it have missed out on it's more likely chances for a playable rep. Stary is from a 5 game series, Custom Robo has the same amount I believe, and Wave Race/1080 combined do (they are so connected, it feels like a Mario and Yoshi series situation with those two). For another retro outside of the three mentioned earlier (I think they are the most likely) Bubbles from Clu-Clu Land I believe is Nintendo's first female protagonist and Chibi-Robo... I actually forgot about the poor fellow; he could really do with some love and he is still relevant. Then there also the more... 'casual' (hate that term) appealing series such as Art Academy and Style Savvy; sold well, targeted key areas, relevant, and have the potential for a playable rep like Wii Fit does.
Of course, I do get your point, and it is certainly harder to establish which series should or are more likely to get in when competing for an unknown but supposedly less amount of slots. Certain key characters are not playable in SSB (King K. Rool, Dixie Kong), certain characters have become key over the years (Bandanna Dee, Isabelle) and the balance of representation is really skewed in favour of some series (Mario, Zelda, Pokémon and Fire Emblem have so many... in part thanks to clones and echoes).
I also realise that Koopaui stole my thunder, by basically saying all this before me but way more concise. Thanks for the mention though.
But there is a lot to pull from. They just aren't super popular or huge new things. At this point, why not pull from small or obscure IP?
But I get what you are saying. You're saying thiere's not a lot that Sakurai is would want to pull from, not what he could pull from. Because there are still, literally hundreds of games and series from across Nintendo history he could use if he wanted to.
Ah; you beat me to it. Nintendo have so many IPs, so many lesser known ones, and it would be fantastic to give them the chance to be known through the marketing capabilities of SSB. I myself learnt a lot about Nintendo's huge library of game characters through the trophies in Melee; though more prominent roles for these series through Stages, Assist Trophies and of course, playable characters, would be fantastic.
When try to break it down to the most likely series to be added into SSB, it sounds like there aren't too many left. It's just that Nintendo hasn't used a lot of those series recently or they just aren't as well known for one reason or another. There are only a few with the relevancy factor, but so many glorious ones to pull from Nintendo's past, and hopefully, give some a future.
I'm always happy to see a more obscure Nintendo IP get the limelight, and I feel I could guess from your avatar a series you'd like to see a playable rep for.