I couldn't choose between pre-existing franchises already represented and not represented. I really want all of the Nintendo all-stars to get in before they add any more 3rd party characters, except for a few exceptions, being Rayman and Banjo. I feel almost all of the series in the game could benefit from a few more characters, and a few first party franchises not in the game should be.
Super Mario has quite a few characters who should surely be playable. Captain Toad, or Toad in some capacity, is long overdue for a roster slot. He's a video game icon at this point and could have a really fun and interesting moveset. I don't see any reasons why he shouldn't be playable, I mean if Isabelle can be beaten up in a fighting game why can't Toad? And there's ways around Captain Toad being unable to jump. And while Waluigi has already been disconfirmed, he would also make a cool character. I mean c'mon, he's the ultimate troll.
The Legend of Zelda is criminally underrepresented in Smash. At this point, only the three characters representing the triforce are even a part of the game in a playable form. Impa is an important side character in the Zelda series and is tailor-made for Smash with her bodyguard status. And there's a handful of interesting villains to pick from. Skull Kid, Midna, and Ghirahim would all be exceptional choices, even if the latter two appear as assist trophies all ready.
Donkey Kong finally got some much needed representation in Smash Ultimate with it's main villain King K. Rool coming aboard. But a very integral part of the DK Country crew is still missing. And no, it's not Kiddy. Dixie Kong has to be in Smash. While Peach and Daisy were busy being captured by their respective kidnappers, Zelda was in Ganondorf's clutch, and Palutena was in dier need of assistance by Pit, Dixie Kong was busy saving Donkey Kong and Diddy Kong, which was a reverse role compared to other Nintendo games where the men saved the women. Dixie's moveset could revolve around her hair and guitar, and would be epic.
Kirby still only has three character in Smash Bros, which is surprising since it's one of Sakurai's series. Some people think the franchise is perfectly represented, but it's one of Nintendo's biggest series nowadays, probably fourth in importance behind Mario, Pokemon, and The Legend of Zelda. Speaking of fourth in importance, Bandana Dee takes that role in the Kirby series. Having him in Smash would be epic, as hes an adorable bada$$ with a spear. Marx and Daroach are two villains I personally want to see playable, and Adeleine appearing would be awesome too.
Star Fox has three characters currently which satisfies most people in the Smash community. Most people think of Krystal when discussing additional Star Fox character, but to me, it would be amazing to see Slippy Toad and Peppy Hare playable. This would mean the four preeminent members of the Star Fox Team would be in the game.
Kid Icarus is well represented in Smash, but it would be cool to see Medusa playable because she would be the first female villain in the game. She's pretty unique and could have a really interesting moveset.
Earthbound is sitting well in Ultimate with two characters. A lot of people want Ninten, but personally, I would prefer Porky Minch. This game is loaded with menacing antagonists, and Porky Minch could potentially be the coolest of them all.
Punch-Out!! doesn't need a second rep, but I will admit it would be awesome to see King Hippo join the fray. He could be another heavyweight in the roster, and function similar to Little Mac. He would play basically as if you were fighting as Giga Mac: a stronger version of Little Mac.
Animal Crossing, Metroid, Yoshi, WarioWare, Fire Emblem, Pikmin, Splatoon, Ice Climber, Duck Hunt, R.O.B., Game and Watch, and Wii Fit are all represented well right now and personally do not need additional characters.
