Dixie Kong- DK's one of the larger Nintendo franchises, so only having two characters is something massively needed to work on, and Dixie has a bunch of relevance to the DK and is probably the 3rd most important Kong.
King K. Rool- DK's main villain, and definitely one of my favorites. I don't think there's much to explain that others haven't.
Meowth- The last villain I really deserves to be in Smash after K. Rool. He's been the antithesis to Pikachu since the beginning of the anime, and continues to be extremely prominent. A cat playstyle hasn't been established in Smash either, making a really interesting new fighter.
Eevee- A pokemon being promoted as Pokemon's 2nd mascot now, Eevee has always had a lot of relevance in the franchise. Eevee's main gimmick, being evolution, already makes it much more unique than any other Pokemon, and evolution doesn't have much to show in Smash currently, so adding Eevee would definitely help with that.
Captain Toad- Toad is the last of the 5 main characters that originated in the original Super Mario Bros, and has continued to have roles in Mario. He also has been becoming more prominent thanks to the Captain Toad games, so I definitely think it's time to see him again.
Waluigi- Waluigi's popularity is pretty much what gets this. He's a super fun character as well, and definitely has plenty of potential with the sports games, and is incredibly recognizable. Let's have him in plz.
Paper Mario- While the series hasn't been going as strong, Paper Mario is the last Mario newcomer I can really think of to be added to Smash that really deserves it. Paper Mario is a large franchise and has a very unique moveset, so having him in would also be super fun too.
Impa- Zelda definitely deserves more characters, and Impa's the most prominent of the non triforce trio. She appears in a ton of games, and has a really unique moveset to add, having her naginata and water bending, while also using her skills from Skyward Sword.
Bandanna Dee- The 4th main Kirby character, who's been used a bunch in newer Kirby games, and has been having as much relevance as Meta Knight and King Dedede. I definitely think he deserves it in.
Elma/Nia- Xenoblade is becoming a larger franchise, and is heavily supported by Nintendo. I feel that Elma or Nia could both do fine as characters to show Xenoblade's success. As for why Nia instead of Rex? Simply because I like Nia a lot more.
3rd parties:
Slime- Dragon Quest is absolutely huge in Japan, enough to where there's a law that it can't be released on a weekday due to too many people skipping work to play it. Slime is also the most recognizable character in it, and definitely has moveset potential.
Jack Frost- SMT and Persona have become really popular games, and Jack Frost is the mascot of Atlus, the company who makes these games. Jack Frost is also used frequently for promotion reasons, both in game and outside of the game, so I definitely think he's an interesting addition. We also don't have nearly enough ice users in Smash.
Banjo & Kazooie- Relics from my childhood that I just can't seem to put to rest. I absolutely love their games, and seeing them in Smash would make me happy beyond belief.
Crash Bandicoot- Frontrunner of the original Playstation and Playstation 2, rival to Nintendo much like how Sonic was to Mario. Having these two rivals in the game would definitely make the internet blow up, much like Sonic's reveal in Brawl.
Phoenix Wright- One of Capcom's bigger franchises, that's always debuted on Nintendo handhelds, and is still getting games. An defense attorney in a Smash? That'd make for one interesting scenerio.