Out of honest curiosity, what makes you so confident? I'd love to see an assist trophy promotion.
It's hard to describe everything, but it's just a combination of alot of Factors, and it's mainly not based of the idea that we will get an Assist Promotion, but more the merits of the Specific characters
Personally I'm thinking either Shadow, Waluigi, or a Combination of Both
-Both are Very Popular characters that are both Relevant, Heavily requested (Especially Waluigi), and would say Iconic as well
-Ease of access since Waluigi is Nintendo, and Shadow is Sega character
-Both already have Models ingame to re-use. Bomberman shows this in effect since his Mii Costume is just his AT Model that's upscaled to the quality of Playable fighters. We also see that Most, if not all the characters in FP2 are re-using models from other sources, perhaps to save time
-Speaking on Bomberman, It shows that Assist characters are being reconsidered in their roles, and I would say Shadow and Waluigi are perhaps bigger deal than Bomberman
-If Papagenos was being true to his word, that Evidence thing he talked about Late July 2020 that pointed toward Geno, Waluigi and/or "Other Unlikely characters" should still be in play. If we are gonna assume that Geno's Regular mii costume was negotiated at the same time as his Spirit, than I would say it implies something more important happening with one or multiple of the 3 other characters this evidence implies of
-Waluigi has also apparently been in the rumor mill also. The sources aren't the most "Reliable" but there is some info there that does suggests some consideration
-There's also been speculation that the lack of Luigi's Mansion 3 Spirits could be because it's being saved for something. This is in assumption that the Spirit board is made AFTER the fighter is chosen, and the Spirit board is not taken into account. Considering that ARMS and Especially Xenoblade Chronicles 2 already had good amount of Spirits, that could be the case
-The Lack of a Villains/Rival tourney with Sephiroth could imply that it may be saved for another character in this Pass that would use this same thing. The counter argument is that there already was a Hero vs Villains tournament back with Hero, but perhaps they'd have an exclusive one with only the Villains/Rival characters
-Sega has not gotten a Costume at all in FP2, Unlike Konami, Capcom, Square, and Namco. Sega (And Namco) also haven't gotten any New made costumes not tied to characters. Square also fits this, but they got fighters in both passes though
If you want to follow the Hype Cycle theory, than these 2 would kind of fit, if you compare them to the Rest of the Pass
-As Assist trophies, they already have some sort of role in the gameplay already, that's on bigger plane than Spirits. there could be the argument that some people might feel scammed that Nintendo held these characters of to take our money (since I know there some people that believe Assists already have majority of moveset completed
-Waluigi is another Mario character, which already has alot of content already in Smash. He's also controversial perhaps.
-Shadow is Heavily Controversial, despite being the 2nd Most popular Sonic character. there's also the fact that some people will still believe that they would be paying for "Echo of Sonic" even if he had the most unique moveset in the game
-Min Min, despite her being a weird start to the pass, is a Franchise that did not have a character already, same with Steve. Sephiroth and Pyra/Mythra come from already represented franchises. If both Shadow and Waluigi were in, they would definitely continue this trend
-As characters from already represented franchises, they are mainly appealing to people already playing Smash now more than characters from New franchises.
-At the same time, if they were wanting to have a big character for around June since FP2 would go over what would be 2 E3's (even if E3 might not be a factor anymore) than Shadow and Waluigi would be Inclusions that would fit the event
-If they are looking to Reveal characters when they'd be most relevant, June is Perfect for character like Shadow since that is Sonic's Official start to the 30th anniversary.
So that's why I believe Either Waluigi, Shadow, or both of them will be 1 or both of the Last FP2 characters, and by proxy why I think we will get an Assist Trophy promotion in this Pass