The Sans Mii costume is probably the best one out there since you straight up look exactly like him ... not just a Mii wearing a costume. That in itself is a huge deal and head nod to both the character and Undertale in general.
To add, Mii Gunner's Final Smash kinda comes close to Sans giant lasers.
Regarding the last fighter left to reveal in the current Fighters Pass, and the additional ones coming afterwards, I think Fighter #5 could actually end up being Doom Slayer. Reason why I believe this is because, aside the DOOM games released for the Nintendo Switch (and the two that are upcoming), seeing the current Pass' selection, Doom Slayer fits the criteria of being a character from a series that pioneered a game genre, has history in gaming in general, and is owned by a third party company. With the exception of Joker from Persona 5; Hero, Banjo & Kazooie, and Terry Bogard each represent games that have been pioneers to some of gamings genres, or mechanics or features within said genres.
In regards to the additional fighters, I do hope they organize them into a second Fighters Pass (Season 2), AND, it'd be interesting to see them release a Bonus dlc fighter that would be Free for a limited time for those who pre-purchase the second Fighters Pass. However... this bonus pre FP2 dlc fighter might end up being a trolly first party character like Piranha Plant. Though, I trust Sakurai could make it interesting. OR, they could straight up make Geno the free bonus pre FP2 dlc fighter (making him Fighter #76 following official roster numbering; #83 counting total individual characters). Now as for the FP2 characters, the ones I believe could end up happening, if Sakurai is going with the idea of pleasing the fans (as stated in the Banjo & Kazooie presentation), would be:
-Ryu Hayabusa
-an indie character (Shantae, or Quote)
-Dante
-Crash Bandicoot
-Rayman
-a Namco rep (Heihachi, Lloyd, or... Klonoa?
)
-Sora