The Case For Frisk:
This is an interesting one. After Sans' inclusion as a Deluxe Mii, most everyone has written Frisk off as impossible or unlikely due to the simple logic of "Why would they not save the Sans costume for when Frisk is a fighter?" Well, I have a counter to that, but first, I want to list what Frisk has going for them, and why Frisk is a lot more possible than you might initially think.
Part 1 - Toby Fox x Nintendo:
Usually, Nintendo has a history of only doing major collaborations with AAA studios. Microsoft, Ubisoft, Bethesda, Koei Tecmo, Sega, Bandai Namco, etc. Even when Nintendo works with other indie developers, it's usually nowhere near the same as other AAA developers. The most they've done is work with Studio MDHR to bring a Cuphead Deluxe Costume and Floral Fury. There is, however, one major exception to this rule, and that is Toby Fox. Nintendo has done collaborations with Toby on the same level as AAA developers. They got Toby on board to compose the entire soundtrack to Little Town Hero, got him to compose for Pokemon Sword and Shield (aka: the biggest media franchise of all time), got him to help on the Mother book by Itoi, and now tying into Smash, included Sans as a Deluxe Mii. Now, you'd think that with all this work, they'd include Undertale Spirits alongside the Sans costume, I mean, they did do this with Cuphead, but nothing. Nothing when the Sans costume was released, nothing when Undertale had it's 5th anniversary, not anything. And this ties in to my next segment.
Part 2 - That's The Spirit... not:
Despite the relationship between Toby Fox and Nintendo, we've had 0 in terms of Undertale spirits. We got some Cuphead spirits less than a month after his Mii Costume debut, so what gives? The same did not happen for Sans, heck, we got 0 for the game's 5th anniversary. It's been over a year, and nothing. The two met IRL, you can't tell me they didn't consider more content. Sakurai considered Toby Fox a "guest" when talking about the Sans costume, why not some measly PNGs? Unless, they're saving those spirits for a DLC fighter pack. And before anyone uses the "Astral Chain got spirits 5 months after the game's release!" argument, 5 months and 14 months are quite the time difference. It happened with Byleth, why not Frisk?
Part 3 - Debunking Arguments Against Frisk:
"The Sans Costume would have come with Frisk!"
This is an argument I see a lot, so let me put it to rest. First, the Sans Costume was added in September 2019. Volume 2 was finalized in November 2019. With how small Toby's team is, I could see Nintendo having the Sans costume in Sept 2019, and then in Oct 2019, contacting Toby for Frisk in Vol. 2. The decision factors that go into each set of newcomers are different. So they could contact Toby for more Undertale content, just like how they seemed out more ARMS content, despite the game already having Spring Man as an assist trophy and Spring Man/Ribbon Girl Mii Costumes.
"Frisk is a Pacifist! They can't have a moveset!"
One, Isabelle. Two, Undertale is not inherently a pacifist game. It does recommend pacifism, but Undertale is all about the choices you make. Maybe you could have Frisk's moveset have a Pacifist and Genocide meter mechanic, which definitely fits in to the pass's theme of making movesets that challenge Smash's control scheme.
"Frisk's name is a spoiler to the Pacifist ending!"
And Megalovania/Gaster Blasters are a spoiler to the Genocide ending. Toby does not care about spoilers, and even if he did, you can just name them "Human" or "Fallen Child" if it's that serious.