*It's my weakest argument, but I honestly think it may have an impact.
Let's go back and look at Smash 4's DLC. Ryu, Cloud and Bayonetta all had the same content and price as the ones that will be available here: a character, a stage and a bunch of songs; all for 6$, of course that’s just the individual Price and not the Wii U+3DS bundles, but it’s not like that will ever happen here. The fact that they are all third-party should probably be small a sign of that.
*Now, let's check what Nintendo themselves got to offer next year on the game department, to see if there's anything to look after: Animal Crossing, Fire Emblem: Three Houses, Pokémon Gen 8, Daemon X Machina and Yoshi's Crafted World. With all due honesty, I beg to see what characters do these games can really offer, but I'm going to cover that better in the next argument.
*Remember when I said I had something to say about Tails that I held back specifically for this? Well, here it is: every character has to offer a stage and songs. Sonic already has not one, but two stages, and decent selection of songs to represent his franchise, hence part of why I don’t see the two-tailed fox happening (though I’d lie if I said there aren't some good Sonic songs absent from the game like With Me or the Crush 40 version of Seven Rings in Hand). If we're going with Nintendo characters or even other character-represented franchises in general, I'm honestly having a hard time thinking about their availability. What can you give Bandana Dee to justify the bundle? Or Porky (which is why I don't see him happening)? Or Sylux, since there’s no existing Metroid Prime 4 gameplay footage as of this date? Hell, let's return to Smash 4 again, who was the only Nintendo newcomer? Oh right, that was Corrin, and he only came with a pair of songs for an existing stage, which is in my honest opinion, what should be done with that kind of newcomers. I can only think of three characters from existing franchises that could offer that kind of deal are Sephiroth (or any other Final Fantasy character, really), Chun-Li and Elma, and that's only because both of their games have more than enough music to let creativity flow through my mind. Also, I didn't think of it back then, but that's another reason why I don't see Rex happening, if they wanted him as a full-fledged character, then they shouldn't have included XC2 songs in the game. Most possible third-party newcomers wouldn't suffer this problem, because there's a lot of material available on those regards. And now that I mentioned Rex...
*If leakers and theories are to be believed (which I do), Mii costumes are compensating for the character's otherwise complete absence from the game. So no Lip, no Rex, no Saki, and so on. Rex's more so, because the whole point of it is to make people feel like they're using the actual character; bringing him later on in the pass would just ruin its purpose.
*This is technically cheating, but I want to bring another fighting game to this: Injustice 2. Way back in 2016, when we barely knew anything about the game aside from some characters,
Ed Boon was asked about bringing guest characters into the game, and this is what he said:
I always think there’s a novelty to it and there’s a certain amount of people who think that when they see a guest character that it took the slot of a DC character that they wanted to have, but you know, Scorpion in Injustice was the most downloaded of all the characters. There was a coolness and a novelty factor to seeing the mixing of the things. Alien and Predator in Mortal Kombat X were the most downloaded of those, so I would be surprised if we didn’t do any of that.
Basically, he says that guest characters are attractive and that they're best-sellers, which is something I agree with. Don't believe me? Go look at what happened with Cloud or the Ninja Turtles, it's clear these characters were attractive to the general public. Of course, we don't have proof to know if guests are as best-selling in Smash as they are in NRS' games, but reactions quite tell who impressed people more. Man, it's weird how no one never asks which DLC character has sold the most, even though it could be a very important factor for speculation and possible additions.
*And then, there's just the overall idea that
Nintendo wants these characters to be "must-buys" and
bring a whole new level of fun. Where's the fun on another Pokémon or Fire Emblem character who solely exists to persuade people into buying their new game entries? Clearly not something people will be interested in after the whole Corrin fiasco.
The only reason why I'm not giving it a 100% chance is because irony is the biggest ***** in life and twists can happen.