Well, Poochy, but he's become such a minor character over the years. In the original Yoshi's Island he was pretty important, but that's pretty much the only additional supporting character exclusive to Yoshi's series.Yeah, back to Yoshi Series reps...really, it's kind of a artifact now that Yoshi is in his own series. He's so close to Mario that there's really not that much for him in terms of characters, and even in Melee one of the Yoshi Stages was from Super Mario World. They should just combine him with Mario, imo.
The only realistic rep I can see is Shy Guy...he's primarily Mario, sure, but is Yoshi's main enemy in all his games, and would be a nice compliment to Yoshi since he comes into different colors. Could probably have a moveset based off the various Shy Guy variants and spin-offs, like Fly Guy, a kart move, a tennis raquet/baseball bat, or something.
Let me think of the important recurring supporting characters for the other game franchises that have not yet been used . . .
Mario: Pretty much all covered aside from Koopalings (Bowser Jr.), Toad, or a group of Bowser's minions (Goomba, Lakitu, and Troopa as a team), Fawful is also probably the most popular Mario bad guy outside of Bowser.
Luigi's Mansion: E. Gadd, King Boo
Wario: Captain Syrup, Mona, Jimmy T.
Donkey Kong: Dixie Kong, K. Rool, Donkey Kong Jr.
Kid Icarus: Medusa, Palutena, Eggplant Wizard
F-Zero: Samurai Goroh, Jody Summers, Black Shadow
Metroid: Ridley, Metroid Prime, Kraid, any one of the other bounty hunters from Hunters or Metroid Prime 3 (probably Weevil since he's an actual Space Pirate and thus the main character of his "Race").
Kirby: Waddle Dee, Prince Fluff, Gooey
Zelda: Toon Zelda/Phantom Zelda, Tingle, Vaati
Pokemon: Tons, but the ones relevant to Generation V going by Nintendo's marketing are probably Victini and Zoroark. Pokemon trainer N with the ability to switch between Rerisham and Zerkrom is also probably a possibility. I'd prefer Victini over Zoroark myself.
Pikmin: Any of the enemies (Louie on a Bulborb?), someone from Pikmin 3.
Earthbound/Mother and Fire Emblem: Too many possibilities to list. Porky, Paula, Poo, Jeff, Starman, Dr. Andonuts, and Mr. Saturn just to name a FEW.
Metal Gear: Liquid Snake, Raiden, Revolver Ocelot
Sonic: Tails, Dr. Eggman, Shadow, Knuckles
I know this is crazy, but personally I'd like to see a revamped Captain N redesigned in the style of Pit's Smash Bros re-appearance. Makes more sense than ROB, to me. You could even revamp the series so the characters are no longer stupid trophies and just part of Captain N's adventures.