Now for the most tricky part, which would be finding two characters from Nintendo's cast that I feel could be added on nothing but popularity within the fanbase. Before I discuss the two I actually did pick, I'll go over some choices that I consider to be runner-ups in this department.
-Normally, Geno would instantly qualify for one of these slots, but alas, Geno isn't normal. After what happened in Ultimate, while Nintendo and Sakurai are likely very much aware of Geno demand, I feel that it's become clear that it's not worth trying to fish the character out of Square-Enix's depths when they have a lot more profitable options than him. It probably hurts some of you for me to say this, but I feel that this is the ugly truth for the character, which is a shame since Mario RPGs as a whole have become a dying species.
-Krystal was a semi-popular Brawl-era request, and she was actually considered for that game as well, but I feel that the popularity for the character trailed off a while ago--besides being an Assist Trophy in Ultimate, Krystal has had some negative stigmas attached to her, between being an easy target for people to make fun of when it comes to certain fandoms, and also being associated with some controversial entries in the Star Fox series (other than Star Fox Zero, which is its own can of worms). That, and I feel that she has perpetually lived in Wolf's shadow within the Smash community as well, since most fans seemed to be satisfied with him as the third Star Fox character in Brawl, and the outcry from him getting cut in Smash 4 eclipsed any outcry over Krystal not making the cut anyways, from what I remember seeing. This could potentially change, however, if the "3D action game remake" project Bandai-Namco is rumored to help Nintendo on turns out to be Star Fox Assault or something similar.
-Characters like Lyn and Hector have been names that have popped up frequently when it comes to more FE characters represented in Smash, but I feel that the hard truth is that unless a popular FE character could serve as an Echo or semi-clone to someone else on the roster, they're going to get passed up for whoever's the protagonist of the latest installment that Intelligent Systems is going to force on recommend to Sakurai.
-Ashley is a runner-up for similar reasons to Krystal, being an Assist Trophy character who has both a notable fandom and hatedom that I feel is overshadowed by something else that fans want more; in this case, I feel that the demand for Ashley has diminished in favor of the demand for more Wario Land-related content in Smash. Ever since Ultimate did barely anything to remedy this issue that people already had gripes about back in Smash 4, I feel that the want for Ashley has subsided for people just wanting anything substantial for the Land side of Wario's series, and I honestly can't blame them. I could probably see Captain Syrup rising over Ashley in the future if Smash 6 does nothing to remedy the Wario Land issue like Ultimate before it.
-Tom Nook is a weird case for me. There's no denying that he's a popular Animal Crossing character, but the one thing that stops me from adding him is Isabelle already being present on the roster. It would feel incredibly awkward if there were three characters that had the Villager framework, if you ask me, mainly just because unlike Fox or Marth's cases, I feel that the Villager should remain relatively unique on the roster, and adding in a second semi-clone for him would make him no longer really stand out in my eyes. Maybe you could cycle Isabelle out for Nook, but I feel that would be more effort than it would be worth doing, if you ask me.
-Out of all of the characters I've listed here, I feel that Bandanna Waddle Dee is easily the "third place" runner-up here. He has a lot going for him that under normal circumstances would make me have easily given him a slot, but under the current circumstances, he's in a weird limbo thanks to Sakurai being very peculiar about how he handles Kirby's content in Smash. It's been discussed to death, but the fact that the Kumazaki era of Kirby games has gotten table scraps so far in Smash makes me feel that the bar for Bandanna Dee is artifically a lot higher than it would be normally. I would love to be proven wrong, but as of now, I doubt Sakurai will budge much with how he treats Kirby content he didn't make or supervise.
As for all the characters from smaller series that I'm not listing here, I feel that the IP revival slot is pretty much dedicated for one of these types of picks. On a similar note, while I like the idea of Diskun too much to get rid of him from my prediction, you can treat his slot as a general "weird legacy character" slot if you'd like.
So, after reviewing all of the major Nintendo universes in Smash, I settled on two characters that I felt were prime-fit for being included solely on popularity; I have decided to choose Waluigi and Porky Minch as these two characters.
-When it came to answering the question "After Ridley and K. Rool, who was the one Nintendo character that had the most buzz about not being in Smash?", it's clear that despite the controversy and backlash that ensued afterwards, Waluigi was undoubtedly the one snub from Ultimate that generated the most buzz on the Internet. The months following Ultimate's reveal, it was practically impossible to go a single day without someone mentioning or joking about Waluigi being excluded from the playable roster (culminating in the Piranha Plant reveal in November leaving everyone confused and bewildered), and this is on top of the character having been a popular in-joke in Nintendo's fanbase for both the years prior and the years following this. Even if Geno took up the mantle of being the hot topic among Mario characters afterwards, his own disconfirmation still had nowhere near the same ripple effect as Waluigi's did. Now, some may wonder why I've put Waluigi on here while excluding Bandanna Waddle Dee, even if it seems that Sakurai is prejudiced against both characters; That's because while I feel that Sakurai is somewhat reluctant to put Waluigi in at this point, especially after witnessing some of the snub backlash himself, I feel that it's still more likely for him to cave in and put Waluigi on the roster, than to give Kirby more representation from the games he didn't direct, sadly enough. Overall, I feel that Waluigi has become the new Ridley, being a character who seems to be arbitrarily excluded from a roster, but has the demand to power through said exclusion anyway.
-Continuing off this metaphor, we have Porky Minch as the new K. Rool in a sense, being a villain who hasn't seen much action in quite a long while, but is still a popular Smash request in spite of this fact. Throughout my time in the Ultimate speculation scene, one of the few first-party names that I kept hearing get brought up in discussion no matter the circumstances was Porky, and that was very much thanks to his peculiar absence in Ultimate, with there not being a lot of Mother antagonist-related Spirits in-game, including Porky himself, and the tipping point here was the Absolutely Safe Capsule spirit using the "empty" sprite for its artwork. In hindsight, I imagine that Porky's boss fight from Brawl was intended to return for Ultimate, but it couldn't make the cut in time, and that's why he is absent as a Spirit. However, said absence made him a very recurring, popular request for Ultimate's DLC, and these requests kept persisting even to the very end of the second Fighter's Pass. The EarthBound/Mother fandom is a very dedicated fandom, and their efforts have worked in their favor a lot in the past (at least, when it comes to everything but getting Mother 3 localized). So in conclusion, I imagine that things can once again work in their favor for Smash 6, and I feel that Porky is thus one of the likeliest "popular demand" candidates for the next installment.