Many characters for me.
First-Party/Second-Party:
Many Kirby characters (Animal Friends, Dark Matter, Gooey, Adeleine, Dark Meta Knight, Daroach, Galacta Knight, Prince Fluff, Magolor, Taranza, Susie, Hyness)
Many Mother characters (Ninten, Giegue, Paula, Jeff, Poo, Porky, Kumatora, Duster, Masked Man)
King Daphnes (LoZ): Ever since seeing him in Hyrule Warriors, I've always loved the idea of a character who turns into a vehicle to fight.
DeMille: I haven't played his game, but I really want to and I'm always down for having more rpg characters in Smash.
Part-Time UFO: I haven't played this game, but since I'm a huge Kirby fan, I have a soft spot for other HAL characters and games. I think a character like this little guy would be super interesting though.
Tank (Trax): I haven't played Trax either, but for those who don't know, Trax is a HAL game on the Gameboy where you could play as a tank and blow things up with different power-ups.
Gator (Pinball: Revenge of the 'Gator): Just because.
Third-Party:
Sinder (Cel Damage): Cel Damage is one of my childhood games and it's really fun and wacky too. Being the prominent character on the box art, I feel like Sinder would be a great fit for Smash. He would also fit in for being a pretty zany character as well and could fight using his car and all the weapons that come with it in the game.
Gunstar Red (Gunstar Heroes): Gunstar Heroes is such a fin game to play and I think the Gunstar Red should be THE Treasure rep for Smash.
Billy Hatcher (Billy Hatcher): Another childhood game of mine. Sega has some pretty neat characters to implement into Smash and I think fighting with a giant egg and chicken powers would be pretty unique.
Aiai (Super Monkey Ball): Another nice Sega character that I think would be interesting as well.
Firebrand (Gargoyle's Quest): Such a cool character from a great little series that was exclusively on Nintendo.
Crypto (Destroy All Humans!): THQ is dead and I think he would have been the best choice for a THQ character. While being quite raunchy, he still would have been a pretty cool choice with all of his alien weapons.
Abe (Oddworld): Haven't played his games yet, but I'm going to so I'm just going to add him here since he seems pretty cool.
Cooking Mama: I only played the first game, but man would it be fun having a cooking based character in Smash.
Maxwell: I really love the Scribblenauts games and his moveset potential is limitless.
Spore Creature: Spore is another childhood game of mine and I'd really love to see a creature fight in Smash. It would be a weird character for sure.
Monkey (Rocksmith): I'm a huge rock n' roll fan and he'd be a cool Ubisoft rep. Plus he'd be a good music/rhythm game rep too.
Tetris Block: Hehehe.
Indie:
Hat Kid (A Hat in Time): A Hat in Time is a great example of a modern 3D platformer so I think Hat Kid would make a really great addition in Smash to fight against some of the bigger platforming mascots.
Hollow Knight: I found Hollow Knight to be an enjoyable game and I think he'd be a pretty cool addition in Smash mainly because I don't think we have a bug character yet.
Cuphead: I really enjoyed his game and he would be a fun and wacky character to have.
Protagonist (West of Loathing): Another great game and really hilarious. I think having a stick figure character in Smash would also be a unique thing to have.
Vice President (Citizens of Earth): I really like Mother and games similar to the series so of course I'd like to have this dude in. His moveset could be based on all the different citizens you can recruit.
Wayne (Hylics): Another game similar to Mother and also a psychedelic title too so he'd be a really cool character to have.
Zetterburn (Rivals of Aether): This game is based off of Smash so it would be really interesting to see him in Smash since he would already have a moveset that fits easily into Smash.
Isaac (The Binding of Isaac): I like this game and considering the amount of power-ups to use in the game, he could have an interesting moveset.
Chicken (Ultimate Chicken Horse): It'd be really cool to see this guy (or any other animal from the game) utilize different stage objects and hazards for their moveset.
Fourth-Party (Impossible Characters):
Ed, Edd, n Eddy: My favorite cartoon of all time.
Spongebob Squarepants: Spongebob Movie the game convinced me.
Jotaro Kujo: Just like what
Dalek_Kolt
said
Yugi Muto: Yugioh was a huge part of my childhood so yeah. Plus Yugioh vs Pokemon pls.
Spiderman: Why not?
Godzilla: Too big, but I'm too much of a Goji fan to care.
Ronald McDonald: I don't know why... but for some reason he'd be pretty cool in Smash.
Michael Jackson: He actually does have some cool moveset potential with his games.
Kurt Cobain: Yeah yeah I know very silly choice, but he was in Guitar Hero so he does have a game appearance.