FWIW, taking a cue out of Watch Da Birdie's book:
- Mario, Zelda, Pokemon are the de facto tier one franchises. They can opt to bend the rules and squeeze in more than other franchises just because of their combined visibility and clout. With that said, I think Mario's complete as of this game unless some break out new character comes about in the current generation's worth of games. Zelda should totally incorporate Impa. Her renewed importance in the lore of the Triforce trio, wonderful design enhancement and her general reception as per Hyrule Warriors (in before "not canon" in a game where Ganondorf performs dive kicks) and Skyward Sword do more than enough to justify her tagging along, especially with her own move set. Pokemon is the one franchise I'll never chance a guess for because so much happens in between generations and trends vary so wildly when considering all the Pokemon available. In my perfect world though, the roster keeps Greninja. Maybe adding a Gen 7 grass type and the "holy grail" Mewtwo would complete the story for Pokemon in Smash. (Fake Edit: I forgot Lucario, fuuuu)
Mario, Luigi, Peach, Rosalina, Bowser, Bowser Jr.
Link, Zelda, Ganondorf, Impa, Shiek, Toon Link
Pikachu, Jigglypuff, Charizard, Greninja, ?, Mewtwo, Lucario*
- Donkey Kong, both by virtue of the series' recent resurgence and as a point of origin for the Mario series could stand to do with one or two new faces. Problem is, do you go for a full set of Kongs (picking one between Dixie and Cranky) or do you bring in an antagonist that hasn't been the primary antagonist in years (K. Rool)?
Donkey Kong, Diddy Kong, Dixie Kong/Cranky Kong? K. Rool?
- Metroid is tough. Being really honest, Ridley is the only other character that would work well enough both for his ties to Samus as a character as well as for being the only regularly reoccuring force she deals with that has proportions that are malleable enough to work into a Smash fighter (see Melee intro and his numerous incarnations vs. Samus). At the same time, if he remained a boss character with the attention to detail Sakurai places on him speficially, then I wouldn't fault that at all.
Samus/Zero Suit Samus, Ridley
- Kirby: Kirby is good. It has it's major players. I'd personally love to see Adeline take the fourth spot in a hypothetical situation but I guess BitF was a bit too correct about her standing versus the main trio.
Kirby, Meta Knight, Dedede
- Kid Icarus as a series saw a complete relaunch with Nintendo's strongest set of characters ever. If the popularity wanes, Pit & Palutena are a great duo and Dark Pit would make for a good starting base for another true "Luigi" style clone (think Lucas, Wolf) as he could run with the remaining weapon types from Uprising not highlighted in Smash 4. At the same time, if exposure and general popularity keeps up, I could definitely see a situation where we get one more goddess (Medusa would be neat to complete the NES trio, but I see Viridi because popularity and she was left in healthy standing post Uprising) and Hades.
Pit, Palutena, Dark Pit (?), Hades (?), Viridi/Medusa (?)
- Fire Emblem is good in terms of how many spots it has, how many eras/moments in its franchise it represents (Marth: the beginning; Ike: the international; Robin & Lucina: the current hype) and how many clearly defined roles each character has (fencer/berserker/red mage or Mage Swordsman). Thing is (and I hope no one thinks badly of me for this), Lucina's probably the first one to go next time around. More than likely she'll ceede her spot to a lord from the next game in the franchise. At the same time, the same was said of Ike sooooo... From a gameplay standpoint though, since she overlaps with the fencer archetype, seeing someone come with a staff, spear, or a thief role would be cool to cover either playstyles or the classic RPG roles
Marth, Ike, Robin, ? (Dark horse: Anna; reasonable expectation: a lead from the most current FE game)
- Star Fox should just get Wolf back really. Beyond that it's good right where it is.
Fox, Falco, Wolf
- F-Zero is a weird beast in that I don't know if it needs a new character given that Captain Falcon is like 10 of every other franchise's characters in how much he is beloved while also being the default face for the series.
Da Captain
- Animal Crossing, Wii series, newer one offs: Probably good for the rest of the series with just Wii Fit Trainer and Villager. Would love to see Monita (Nintendoworld) just because her design lends well to another one of Smash's "exotic" (re: wtf) style characters but I'm not placing any bets on it. Same goes for the Wonderful Ones given how W101 performed.
Villager, Wii Fit Trainer, Pikmin & Olimar (with more captain alts), Little Mac, Shulk
- Earthbound: #FreeLucas
Ness. Lucas
- Retros, one offs & new franchises: Of course I'm putting my boy Takamaru at the front. As of now, he's the only one of my wildest wishes that has a chance anymore thanks to that main character/representative and franchise potential comment in his recent interview. (R.I.P Mike Jones & Lip). I feel like Splatoon is a very early contender for an entrant especially with how many boxes were ticked with the reveal (new character driven IP by EAD given a big presence at a trade show with an immediately "Nintendo" character design). And as always #FreeIceClimbers. The large majority of interesting one offs that make sense are already in the series
Game & Watch, R.O.B., Ice Climbers, Duck Hunt Dog, Takamaru, Inklings, Wario (as much as I'd love Captain Syrup to absorb the Wario Land stuff while Smash Wario did his WarioWare slanted things, that's looking more and more like a no), Yoshi
- Third Parties: Sonic, Pac Man and Megaman should be tenured. That's a pretty perfect trio all else considered. We like to joke around about SE and Konami but really, even if they never contributed a character, I think the guest list is set. Black Mage and Snake/Bomberman would be pretty nuts though.
Sonic, Pac Man, Megaman
Keep in mind I didn't account for roster bloat though. I could definitely see the next Smash be either the biggest (but not by a big number, instead swapping parts) or a point where the series decides to keep a hard number for every entry afterwards since working on a MUGEN level roster isn't the most feasible of things.