There was a similar list that I found recently, so I was already cooking up a response to that when I stumbled across this. I'll toss in a few characters that I'd like to see in the next Smash, and a few that I don't think will make the grade.
Yes - Mipha (The Legend of Zelda: Breath of the Wild)
The Legend of Zelda needs some more love. Melee, Brawl and S4 have seen, for the most part, the same five characters repping the franchise covering many different styles from nimble and weak to slow and mighty. With Sheik having not been in a Zelda game since '98, and having been completely left behind in Breath of the Wild's stories of the Sheikah, it may be time to move on from her. An ample replacement? You could fulfill the fast niche and toss in some fresh gameplay ideas by introducing Mipha. A supporting character from one of the best video games ever, Mipha could combine long-range, quick spear strikes (spears in smash? Interesting.) with Greninja-esque water manipulation to create a graceful fighter that would be an interesting departure from the kind of characters to which Smash usually plays host.
No - Konami characters
A Konami IP in Smash? Damn, I sure wish. However with how tightly Konami has been clinging to their IPs as the company drowns further and further in a sea of mobile games and pachislots, It's unlikely that Nintendo ever gets rights to these characters. This is especially true in the case of Snake, as Sakurai has made a case in the past to not go ahead with MGS in Smash without word from his good buddy Hideo Kojima, the series' creator. Unfortunately this Konami assholery looms over Castlevania as well (even with the new Netflix anime series which, funny enough, is actually pretty good), so who knows if either IP will see the light of day again.
Yes - Decidueye (Pokemon series)
Talk always circulates about which Pokemon in a recent generation would work well in Smash Bros - after all, every Smash game to date has at least one playable mon from the at-the-time current generation. So, talking about Gen 7, who's most fitting? Why not Decidueye! You could take this character in plenty of directions: he's a ghost, he flies, he fires arrows from his damn arms, and Pokken Tournament DX gives inspiration to create his character in Smash Bros. Not to mention, Smash 4 was sorely missing a Grass-type playable character after Ivysaur was sidelined. Perfect time to give Decidueye a shot (no pun intended).
No - Goku...or anyone Dragon Ball for that matter
Although he may not be in charge of the next Smash game, Sakurai's on the record as being starkly opposed to any Dragon Ball character, especially noting how the manga and anime came before the games, and that such an addition would be akin to "a cartoon character". Dragon Ball isn't a gaming IP by any stretch. If you want to beat ass with Goku, I strongly recommend Dragon Ball FighterZ when it comes out in January of next year. As someone who played the beta, I can attest that you'll probably love it if you play Smash.
Yes - Isaac (Golden Sun)
Nintendo is no stranger to giving second chances to series that had a terrible outing in sales; we wouldn't have Robin, Lucina or Corrin were they not so forgiving. (We could do without one or two of those, though.) Not gonna pretend like I've played a lot of Golden Sun, so I don't know too much about what would constitute a moveset for Isaac, but Nintendo's RPGs are all treasures, and they need more attention, especially those from the 2000-2009 era as both the GBA and DS had libraryfuls of godlike role-playing games. So, uh, yeah. Isaac for Smash?
No - Ice Climbers
Bah. It pains me to put this one as a no on my list given the hours I spent playing them in Melee. (I'd play them in Brawl, too, but Wolf was a thing.) But they who were once a celebrated character that celebrated the early NES days have been largely forgotten and, sadly, they have been effectively replaced completely within the series. The tag-team/puppet character archetype has been taken over by Rosalina, who will always be more popular because she's a Mario character, while the retro-NES representation has been fulfilled by Duck Hunt. Were it not for the 3DS's hardware limitations, we might have been talking about how cool they were in S4, but Nana and Popo have been lost to time - and if you ask some pro Smashers, that might be for the best.
Yes - Hector (Fire Emblem)
Even with Nintendo's obsessive pushing of Fire Emblem Fates, fans have voiced loud and clear that the likes of Ryoma and Camilla could never trump a legendary character like Hector. He has been a long-desired character for Smash, and now thanks to Fire Emblem Heroes there's a greater pool of content upon which to base the Ostian Badass's would-be appearance. It would take work to make him feel unique, as he faces inevitable gameplay similarities to Ganondorf (and to a lesser extent King Dedede), but swinging around a ginormous axe in the midst of an all-out brawl would make for some entertaining and ridiculous moments.
No - Corrin (Fire Emblem)
Our favorite dragon waifu gets the honor as one of the least likely characters to return to Smash along with Dark Pit. There were a number of problematic gameplay issues associated with her, which amounted to a character that stomped online battles with very little learning curve. It doesn't help that we're getting REALLY tired of seeing Fire Emblem Fates dominating the series' now-plentiful spin-off media. Corrin's gameplay style and niches can be accomplished by other characters and ideas, so it's time to get rid of her to cut back on the Fire Emblem rep, if only so that we can make way for Hector.
Yes - Heihachi Mishima (Tekken series)
Heihachi was actually planned to be included in Smash 4 DLC alongside Ryu but his moveset proved too difficult to implement into the game. With numerous experiments made by smash (and other fighting games) to include weird and wonderful characters, it's possible to recreate Heihachi within a 2D environment maybe in a similar vein to Street Fighter X Tekken. You could even include his son Kazuya instead (perhaps even Jin, his grandson and the current Tekken protagonist), and you'd have the fans appeased as all three play pretty similarly, but Heihachi is the obvious choice over the other two given his legacy. It's just, good luck doing EWGFs on those controllers...
I might add more later.