In Terms of Assist Trophies getting in
I do think there are Pros and Cons to an Assist's Chances of Playability, and these Pros and Cons also depends on the Characters
Pros
-Assists already have Models to Re-Use within the Game Itself. Characters like Skull Kid, Isaac, and Bomberman have their AT Models Re-Purposed into Costumes. And while not an Assist, Chrom uses his Final Smash Model as his Playable Character model.
-Because they are already in the Game with a Full Model, all the Licensing for them has been done before. While Licensing for an Assist and a Character is not the Same, It's a Great headstart, compared to a Character that's not represented at all. This also doesn't take into account good amount of Popular AT are First Party
-People actually want to See Some of these characters as Playable. that's actually a Big Part of why Certain Characters are Assists in the First Place.
Cons
-There could be an Issue of Nintendo and/or Sakurai thinking that Their Presence as an Assist Trophy is Enough for that character, so there's not as Much of an Incentive to make them Playable, since they are represented well enough
-There's also an Issue of Perception that people are getting ripped off, that they could have made the Assist Playable this Entire time, and them charging 5 Dollars to play as these characters is Nintendo just trying to rip people off
The Most Likely Assist Promotions in My Opinion are Waluigi, Shadow, Isaac, and Shovel Knight. All of them have their Pros and Cons over other Characters. I also don't believe Waluigi is the only Possible Promotion. He had the biggest wave with his Deconfirmation, but that doesn't discount the Backlash and/or Disappointment Shadow and Isaac got, and while People were excited Shovel Knight was in the game at all, there still Many people that want him Playable
Waluigi and Shadow are both from Represented Franchise, though I think Shadow has it much better in this regard since
-Sonic only has 1 Character, While Mario has 9 Whole Characters
-Sonic only has 2 Stages, While Mario Has almost 20
-Sonic has 20 Music tracks, Mario has like 100, or even more
-Sonic also hasn't gotten a New Character or Stage at all throughout Ultimate, Mario has gotten New Stage and Has gotten 2 New Characters
Isaac and Shovel Knight are Completely New Franchises, So they got that taken care of
It could happen, It could not happen