First Party Characters
Dixie Kong (Donkey Kong Country)
Positives: "Relevant" due to the resurgence of Donkey Kong Country in “Returns”, one of the most popular DKC characters back in the 90s. Has unique moves separating her from DK and Diddy.
Negatives: Really? Not that much.
King K. Rool (Donkey Kong Country)
Positives: The primary villain of Donkey Kong Country's original three games and it's TV show. DKC was explosively popular on the SNES.
Negatives: Hasn't been seen (outside of Smash) since Donkey Kong: Barrel Blast in 2008.
Neutral: The fact that he would be another heavyweight villain could be seen as either a positive or a negative.
Isaac (Golden Sun)
Positives: Comes from one of the best selling games on the GBA (it competed with Mario, Pokémon and Zelda), there aren't really any GBA-specific representatives (Toon Link and Zero Suit Samus are about the limit, but they aren't associated with the GBA). Oodles of moveset potential.
Negatives: Hasn't been seen since Dark Dawn in 2010. Has minor overlap as a “Red Mage” Fighter with Robin.
Ashley (Wario Ware)
Positives: A popular character from WarioWare. Pure magic users are rare in Smash, so she'd definitely be unique.
Negatives: Moveset potential might be a minor hurdle (but it's there).
Third Party Characters
Being a third party is both a positive and a negative, the hype from their inclusion could balance out the rights issues.
Simon Belmont (Castlevania)
Positives: Castlevania was one of the defining games of the NES, and the series is still around today. After Snake and Bomberman, he's probably the next in line for a Konami character.
Negatives: Nothing much, really?
Geno (Super Mario RPG)
Positives: Frequently requested according to Sakurai, and has some moveset potential as a Gunner character.
Negatives: Has only been in one SNES game, which wasn't even released in Europe until the New 10s (and since that release was on the Wii U, that means not many people would have bought it even then).
Crono (Crono Trigger)
Positives: With Takamaru and Lyn gone, he might be our last hope for a 'Samurai'/Katana-wielding character (unless you want Ryoma, but really, why would you want Ryoma over CRONO?)
Negatives: Chrono Trigger, while a much beloved SNES game, hasn't been seen since 2008/2009.
Miles "Tails" Prower (Sonic the Hedgehog)
Positives: One of the most well known Player 2 characters and Sidekicks in Video Game Culture. Has unique traits that make him different from Sonic.
Negatives: Like Dixie and Simon, not really all that much?
Shadow the Hedgehog (Sonic the Hedgehog)
Positives: An easy Semi-Clone of Sonic, being so similar in stature and attributes. Sonic Adventure 2 was the first cross-platform Sonic game.
Negatives: Has competition from Tails, who is arguably more 'worthy' of inclusion.
Ken (Street Fighter)
Positives: An easy Echo Character for Ryu. It's, like, super obvious.
Negatives: Would Sakurai feel justified making a 3rd party character an Echo Fighter?