That's true, but I think we're at that point with all of the franchises in the current roster, except perhaps DK and Metroid (and that's arguable since apart from Ridley or Dark Samus who do you even do). Most of the characters they could add at this point are from less popular/more niche franchises or games that have yet to get playable character representation. And in general that is what we got a lot of with the Smash 4 additions.
In terms of already represented franchises, Wario Ware has existed for a bit over 10 years and already has had six games released in retail so far (7 if you count Mega Party Games). I've said before that the only thing holding more Wario Ware representation back is that it's hard to prioritize one character over the other, it seems Ashley is more popular in Japan, people like her in the West too but I also see a lot of request for Jimmy T and Mona as well. But despite that, Wario Ware is probably the franchise in Smash with the most room to grow, even though people seem to overlook it a lot during character requests.
I'd actually say that Animal Crossing is in a slightly similar position. It's released almost as many games as Wario Ware and has great popularity as well, but it becomes difficult to decide who a newcomer should be. It's also hard for me at least to see how to differentiate the characters' movesets, but I could see Mr. Resetti using his pickaxe, just as an example.
In terms of unrepresented series, you have your stuff like Advance Wars, Golden Sun, Custom Robo, etc... Stuff that gets a lot of requests but hasn't been relevant in the current gen. If Isaac continues to get support on the poll, I can see him getting in, but Andy/Sami and Ray have an uphill battle.
So where does that leave unused Nintendo IP? I think Rhythm Heaven makes a lot of sense, we know Chorus Kids were likely going to be in the game, and Nintendo is still actively supporting that franchise, with a fourth game in the series coming to Japan soon. If Splatoon catches on, a character can be used from there as well. One series that is largely ignored in terms of roster requests that is still active from Nintendo is Style Savvy. I think their third game has already been released on the 3DS, and that's a series that started in 2008 and sticks to handhelds. If Sakurai is struck with inspiration, I can see him including Style Savvy representation as an oddball inclusion for the next game, much like how Wii Fit Trainer was introduced in Smash 4.
And of course, we'll probably get a random retro inclusion in the next Smash as well, but with Duck Hunt already on the roster, who will Sakurai choose? Takamaru? Lip? Ayumi Tachibana? Although I'd be interested in both of their characters, their history of being primarily Japanese-only may give them difficulty in being put on the roster. But I'm having trouble thinking of other wacky retro characters that could be included, so it'll be interesting to see what direction, if any, Sakurai goes with for retro characters next go around.
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