Let's have a look:
6 Super Mario characters.
5 Pokémon characters.
4 Legend of Zelda characters.
4 Donkey Kong characters.
3 Kirby characters.
3 Starfox characters.
3 Wario characters.
2 Kid Icarus characters.
2 Earthbound/Mother characters.
2 Yoshi characters.
2 Fire Emblem characters.
2 Metroid characters.
and some One Representant series.
This is the roster I can imagine to be the one of SSB4.
First off a few questions I'll answer to give you an image of why I consider this or that franchise being represented by x characters. I'll also put in some numbers to illustrate some arguments.
Why does the Donkey Kong franchise deserve 4 spots?
The Donkey Kong franchise counts 11 games of the main series(DKC, DK Arcade and DK64) and 13 spinoffs which makes a total amount of 24 games. The Legend of Zelda counts 25 games.(17 from the main series, 7 spinoffs)
DKCR has sold 6,30 million times on the Wii while Skyward Sword only sold 3,67 million times. (comparing the most recent games of the main series and not considering A Link Between Worlds which has around 0,86 million sold copies until now and is only out for a month yet)
I could also take examples from the last official Kirby game from the main series (or from Metroid, Fire Emblem,...); we would get even a huger difference in terms of sold copies. Therefore 4 spots would be absolutely plausible.
Also we have to consider the fact Brawl came out before DKCR. Nintendo couldn't know already right from the beginning that DKCR would sell that well.
One representant may be a tribute to the old games(King K.Rool) while the other would be used to promote DKC:Tropical Freeze which is released before SSB4. (Cranky or Dixie, but to me Cranky seems more interesting as character)
Why 3 Wario characters, 2 Yoshi characters and 2 Kid Icarus characters?
From Melee to Brawl we had a few One Representant series like Kirby, Donkey Kong, Metroid and Earthbound/Mother that got a 2nd representant (and Kirby even got a 3rd too), so it is pretty likely that we will have more One Representant series that may get a 2nd representant. The only ones that could offer enough characters would be the Yoshi series, the Wario series, the Kid Icarus series and the F-Zero series. However talking of F-Zero, a pretty inactive series until now and maybe even for some time in the future too, there is no character from F-Zero who would be a good reason to buy SSB4 or vice-versa buy an F-Zero game beside Captain Falcon. Captain Falcon stays THE face of F-Zero and there is no second one. (no Samurai Goroh, no Black Shadow and no Judy Foster)
The Wario series sells quite well, even though Game&Wario is until now not on the same level as most other Wario games. The Wario series also counts many games and even has two main series: Wario Land and WarioWare. In order to not give the priority to either Wario Land or either WarioWare, it makes sense to give a Wario Land character a spot in the roster and a WarioWare character a spot too. While Captain Syrup seems to be the most important character besides Wario in Wario Land(considering her appearances), for WarioWare there are a few characters that could be added. However as we got already a Fitness trainer, why wouldn't Sakurai want to add a blue-afro-wearing dancer as unique as Jimmy T. is? And I go with Kamek for the Yoshi series so a witch like Ashley wouldn't be a good addition when we get already Kamek. And Mona,... Well. I can't think of any interesting moveset that could work and comparing Mona to Jimmy T., Jimmy looks more interesting
Now let's come to the Yoshi series. The last game from the main series, Yoshi's Island DS sold 2,71 million copies which is better than the last Kirby game sold. Therefore a 2nd representant should be taken into account. Who would be the best option for the 2nd spot of the series? Baby Bowser? Well... Kamek makes more appearances in the games and seems to be the main villain, so it seems more logical to put in the real nemesis of Yoshi: Kamek!
Now let's come to Kid Icarus. It's a franchise reborn thanks to Sakurai on the 3DS with the title Kid Icarus: Uprising. In order to push the game with the release of SSB4, he will need to add another character. Otherwise some SSB-player may think Pit is still a retro-character and so they won't really notice that there is a new game of the series out. Why Medusa? While most people here think the new character will be Palutena, I totally disagree as I feel Medusa being more interesting and being nearly as important as Palutena. Of course, the japanese name of the franchise contains Palutena in it, just as The Legend of Zelda contains the name of Zelda. However I don't think this is an argument that can eliminate Medusa right away. Remember Kid Icarus: Uprising? Medusa will come back, so don't think you have left her behind you
Why did you cut Ike and Snake?
Well, if Sakurai likes Ikes moveset, he can transfer it to Chrom. However 3 bluehaired swordsmen from the same franchise looks a bit strange, doesn't it? Also I think two characters are pretty representative for Fire Emblem. One being the legendary Marth who is the most important lord of the series anyway. And the other being always a character, most likely the lord, of the most recent Fire Emblem game. This is a good way to avoid adding too many swordsmen over more interesting characters and still the recent FE games get supported by SSB4.
Why I've cut Snake? Well, Metal Gear is barely present on Nintendo consoles, so I don't see a need to keep him in the roster anymore. As a guest character for one game he was okay, but that's it.
Punch-Out!, Dillon's Rolling Western and Takamaru?
Punch-Out! reborn on the Wii needs to get more attention if Nintendo wants to release in the future again another Punch-Out! game. To keep the attention on the protagonist of the series, Sakurai has to put Little Mac in the roster. Also we saw already a stage being a boxing arena and this perfectly fits for Little Mac, just as the Galaxy stage hinted Rosalina. Beside that, Little Mac is one of the most wanted newcomers, he is on so many lists and in so many rosters. Even Sakurai can't miss this wish to get Little Mac in SSB4.
In the Super Smash Bros universe, we have no character who symbolizes the japanese history: the samurai. Nintendo has an eShop title called Sakura Samurai which has the historical japanese setting of samurais. Beside that one, Nintendo has Takamaru, a good retro option for SSB4. In Nintendoland there is even a game called Takamaru's Castle. We can be sure Nintendo hasn't forgotten who Takamaru is/was and therefore he seems also a possible character choice for the samurai representing the samurais of the East.
Speaking of East, let's got to the West. Dillon's Rolling Western plays in the Cowboy/Ranger setting we haven't seen yet in SSB4. The eShop franchise counts two games in almost 2-3 years and is even supported by Nintendo by sharing a Mii plaza puzzle on the topic of Dillon's Rolling Western. Nintendo has all the rights for the game and the characters and therefore there are no real obstacles that could stop the rolling armadillo from entering the roster. I feel like Dillon would be a good addition because of the setting, but also because he has moveset potential and can be very unique.
I think I've talked now on the topic of every newcomer I've added and on the topic of every cut I've made. I'm fine with my roster. I like it as it has many different characters from different franchises.