Honestly, that leak does actually make sense to me (UT Rep + Waluigi), and while I'm going to say I'm neutral until we get solid evidence that this leak is fabricated, the reveal of somebody other than those two, the reveal of Sans, or the reveal of Waluigi after Frisk/Papyrus (the leak said it won't be Sans more than likely), I can see where Waluigi and the Undertale character come from.
Waluigi is, believe it or not, massively popular among casuals by mere virtue of being a Mario character; I really think people in the Smash speculation scene don't understand just how well-known Mario spin offs are, and how beloved they are by casuals worldwide: Waluigi is in one of the best selling Mario games ever, Mario Kart Wii. And, to be honest, when it comes to something as legendary and popular as Mario, I'm not quite sure Sakurai would have the same apprehension as with Corrin. Plus, while it would be a waste to scrap Waluigi's Assist Trophy, there's plenty of ways around it (disable the AT when Waluigi is played as, put AT Waluigi in a new outfit, replace it with a character who can do what he did, and there might be some solutions I didn't think up).
Personally, I think that even if you don't like, or even outright hate Waluigi (unless he is your least wanted by a country mile and a half), you have to admit the prospect of a character who effectively disconfirms DISCONFIRMATION AS A WHOLE (killing AT, Mii Costume, and Spirit disconfirmations in one fell swoop) would be very enticing to the Smash Bros. fandom, who would probably buy him en masse to prove to Nintendo they want more Assist promotions and Spirit promotions, while Waluigi practically sells himself to casuals.
Meanwhile, we have Undertale. I've previously discussed this at-length for the RTC thread, so I'll keep it short. I think I might have new information to go off of.
- Undertale is extremely popular and iconic for an indie game, being one that will shape the lives of the next generation.
- All of the DLC characters so far cater to old-school gamers (Three out of the four variants of Hero, Banjo & Kazooie) and adults (Luminary, Joker): there are no characters who are popular among children. Undertale would easily remedy this.
- This may not be true in regards to Japan, but at least seems to be accurate for the United States. Anybody who wants to contend this point can.
- The relationship between Nintendo and indie developers has become one of the things Nintendo wants to push the most, making it likely that Undertale made the list.
- Toby Fox is a known fan of Smash Bros., as he was more preoccupied with the Ultimate teaser than the teaser for his own game, and Deltarune includes a reference to Smash Bros.
- As an anecdote, another series that would get a fighter in Smash, Persona (specifically Persona 5 sub-series), includes a reference to Smash Bros. as well (unless this is fabricated). However, that doesn't mean it's an intentional hint.
- Sakurai is familiar with Undertale and has played it.
- As for why it wouldn't be Sans... (limiting myself to just Frisk and Papyrus).
- Frisk is the primary hero, villain, and protagonist of the game. Sakurai may have been enticed by a moveset that could represent all of Undertale as opposed to only one third.
- The Human has a back-up name to go by if Toby Fox does not want spoilers, though given Groove Coaster I won't hold it against Sans too much. Sans' primary combat abilities are spoilers.
- Papyrus can do everything Sans can, sans Gaster Blasters, while these traits are not spoilery.
- The most likely Undertale stage, the Last Corridor (official name of Judgement Hall), could feature Sans in the background, or even as the main element of the stage.
- Other Undertale stages could feature Sans as an NPC.
- It's possible Sans is among the DLC Spirits and Mii Costumes.
- While Sans is more popular than Frisk and Papyrus, the gap is not that big, and all of them are very iconic and memorable characters to many people (Frisk is contentious about this, but Papyrus certainly is not that far away from Sans). Sans is simply the most memetic.
Even if the leak itself is fake, then I still believe at least Undertale is not off the table, and it would be very, very easy for Toby to just say "no lol i'm not involved with smash. just a fan" or something to that effect. I don't recall this ever happening, though. Waluigi is more contentious, but hey, everything's a rule until it's broken.
I'm very not certain in the idea that the Fighter's Pass, barring leaks, is a "one per company" deal, mostly because it feels like something that'd only be done to complete a quota and it doesn't sound like Sakurai cares about quotas. It's not a rule, but more likely than not it could come to pass that it's one character per company. One per series, on the other hand, is even more likely to be true because then you're not forging new ground, and as far as we've seen, second uniques are very unlikely as there seems to be a preference for new series, though not impossible until somebody says something.
I don't have anything to say on the "female with a projectile" other than if it were... you know who... I'd be a happy camper. But given the guy seems to be Japanese, he (and his source?) would know who Arle is. (Also, in the next game/pass I would say Arle's easily the likeliest puzzle game rep hands down, but right now I think Layton has a better argument. If Jibanyan gets in, though, Arle will be the likeliest again.) Spreading my wings so far that I can't talk about the current point.