I would argue that Sakurai wasn't even aware of King K. Rool's demand during pre-Smash 4. The last poll Sakurai conducted before that game was the Brawl poll in Japan which gave us two newcomers in Sonic and Mega Man, who was
directly below Sonic in third-party choices*. Considering that Diddy Kong hadn't been confirmed yet and the DK fan base was focused on getting him in, I doubt K. Rool performed well. Seeing as Sakurai used this same poll for Smash 4, he wouldn't be able to tell that K. Rool was actually wanted and thus had no reason to put him in. Then the ballot came and while it is true he didn't get in again, he did receive a consolation prize in the form of a Mii Costume. This is the first time K. Rool had ever received recognition outside of a trophy. Considering that he acknowledged Geno as a popular request, whom's only appearance was a popular SNES Mario RPG and a cameo in GBA's Mario Superstar Saga, I guaranteed you that K. Rool had to perform strongly to receive additional recognition.
I don't think they can afford to just ignore the most popular requests and add whoever Sakurai wants that happens to get some amount of votes. The ballot was not a suggestion box and there's no doubt that Bayonetta won among realizable characters. Outside maybe Banjo-Kazooie and Ridley, who else was unrealizable that couldn't have gotten in as DLC that game? If those are it, then it's not implausible for someone like K. Rool to show up among the Top 5, especially when the Kremling Kampaigner got the recognition of Retro Studios and K. Rool's original creator Steven Mayles. Considering that we got 1.8 million votes in the Ballot, that's potentially hundreds of thousands of fans that showed up to vote for him.
If he placed right below Bayonetta like I'm thinking, he will get in recent appearances be damned. If for some reason he didn't, then he probably won't (but I still think that a new Retro Studios' DK game announcement will come in time with Kremlings to get him added as DLC anyway). But except for the Inklings, I can't think of anyone who could have. So it's really just a matter of learning if Inklings were a ballot runner-up or not. I'd also argue that "unrealizable" characters like Ridley and Banjo have a distant shot at this game but that's for another day.
*: personally I think Mega Man was the highest requested character that didn't get into Brawl outside of possibly Ridley and Villager (whom's fan base from what recall collapsed after Brawl in that country). Of course I have nothing to support my claim but I can't imagine there were a ton of first-party characters that could have been picked above him. Maybe like two tops?