I wasn't referring to the Rathalos Armor set, since I would consider that to be connected to Rathalos, but rather, the Hunter's Helm and Hunter's Mail Mii costume.
Not gonna lie I forgot these were a thing. Yeah, I beleive the Hunter's Gear is just early-game gear from before you have monster parts to make stuff with. That's pretty unambiguously the Hunter. So even if there aren't Spirits, the Hunters
are represented in some form.
I think the main thing about this is that favoring playable characters doesn't necessarily mean that they always go for them above other options. There are some cases where it makes more sense not to go with the player, or sometimes the mascot isn't the player, and uh..."skips the line" for lack of a better phrase.
Most of the reasoning behind each choices popularity is pretty easily explained. People talk about Sans over Frisk because the former is a mascot, and the latter is not only an avatar...kinda, but have a very nebulous mechanic attached that would be insanely difficult to translate to Super Smash Bros. Monokuma similarly is also a mascot, appears in the series more often, is more easily envisioned as a fighter, and is up against a milktoast protagonist to boot. The popularity contest really isn't fair here. Freddy over Night Guard is no contest. He's the mascot for one, but you're also pitting him against a character who has no abilities, character model, or even a cannon design in the games other than the pixelated sprite he has in Sister Location. You'd get much farther trying to advocate for a character like Gregory, though he would likely also come with at least a version of Freddy. Solaire over Chosen Undead I think is just a case of people not wanting an avatar.
I can't really comment on Diablo or Elden Ring since I know nothing about the characters.
As far as why people fixate on the Monster Hunter. I'm not entirely sure. They are an avatar, which usually means they're the least popular idea, but maybe they're kind of seen as the only idea like Steve was for Minecraft. The hunter has a ton of moveset potential, and is easy to conceptualize, the monsters are huge and hard or impossible to downsize for a fighter. There's the idea of them adding a Felyne, but it's reletively new, and proposed after the concept of the hunter has taken hold.
And this is just a personal thing, but the idea of adding the Felyne instead of the hunter feels weird to me since, to me, someone who hasn't played the games, the hunter just feels like a better choice, and would feel better to play, even if the Felyne would technically be more interesting since they'd be a shorty with heavy weapons. It's also just...why pick Felyne when there's nothing wrong with Monster Hunter? It feels kinda arbitrary. I'll accept it if it's what ends up happening, but conceptually, it just feels kinda weird.
Adding to the "Smash favors the playable characters" thing, when you focus solely on the fighters we already have, there's a couple grey areas but I don't think there's anything
too rule-bending? The most glaring exception is Pokemon, but you
do play as the Pokemon themselves in combat, issuing commands that they carry out. So while Pikachu isn't
the player character, you can still control him to some degree, so it's not entirely unprecedented. ROB also an exception; He's a periphal who inputs commands for you, so technically he's the
opposite of a playable character. I don't know if he even
shows up in his games. He's not just a mascot, he's a
piece of hardware, which is a very unique situation in the context of Smash.
Rex is the player character of Xenoblade 2 and not Pyra and Mythra, but they
do fight as a team. Also we know for certain that Sakurai wanted Rex originally, but ran into technical difficulties (Probably exacerbated by the DLC being made by a skeleton team and having to port over their XC2 models). Similarly, Slime was a Plan B for if Hero was off-limits.
So, Smash doesn't
always choose the primary player character, but it's undeniable that they have a strong
bias towards playable characters. This is even if they have mascots, like how we got Villager before Tom Nook or Isabelle, Joker over Morgana, and the aforementioned Hero over Slime. And as I've said, Pikachu is a notable exception, but unlike the other examples given, he's still controllable to some degree. So while exceptions are possible, precidence leans towards playable characters, more often than not.
For the examples given? I will say there's a possibility we get a Danganronpa MC over Monokuma, given the precedence, but my knowledge of the series is slim and I can't say much more than that. With Dark Souls and Elden Ring, the Chosen Undead and Tarnished are still discussed quite often. The most beloved NPCs and bosses are worth noting, but we shouldn't discount the avatars entirely. Sans is a unique case because of how virally popular he is, and like Smash Puppy said, Frisk has some logistic issues in representing the "Spare" mechanic (Though like with Dangan, I don't know enough to give a real input on that). Same with Diablo. And like you said, we don't really see the FNAF player character, so it's hard to do anything with that.
So to circle back around to Monster Hunter, I won't say it's the
only shot we have, but I disagree with people acting like they're definitely off the table. The Hunter's lack of appearance in other media
is notable, yes, and MvCI is the sole exception, yes. However, the fact there's an exception at all proves there isn't a
hard rule against Hunters being in crossovers. The old MvC3 interview is still passed around, but we've already established that's contradicted by MvCI, and any rumors surrounding that game are only rumors and shouldn't be taken as fact.
Is there a chance we'll have to settle for Palico instead? Yes, I'll admit that. They've been used in crossovers way more than the Hunters, so even if Smash trends towards player characters, we shouldn't count them out entirely. But on that same note, there's no definitive hard rule against the Hunters being playable either, and we shouldn't discount them because of that, especially when Smash trends towards whoever's playable (
Trends, of course, not
only players). (and I've already gone over the logistic issues of adding a whale-sized dragon...)