Are you calling Incineroar or Richter Fan favorites? Last I checked they weren't that big in the ballot (Incineroar wasn't even known to the public at the time). Also, Assuming Sakurai's Adoration of Minecraft is anything to go by, I feel Steve is more likely (because Sakurai bias). Like you said "Whatever Sakurai wants, gets in"
Besides another factor in choosing characters is what new stuff they will bring to the table. Steve can bring a lot more unique stuff with his massive amount of weapons, placing blocks, and even possible crafting. Not saying that Banjo Kazooie wouldn't bring anything new to the table... i just feel like they would retread a lot of stuff already in Smash... An Animal Bird Combo? Duck Hunt's Got you covered. 3D Platformer Representation? Mario and Rosalina Say Hi.
IDK, it's just the more I think about it, Steve seems more likely.
Incineroar is obviously different, because when the roster was being made in 2015, he wouldn't know who from Gen 7 would be a fan favorite because it hadn't been released yet. And Richter is an echo, who he specifically mentioned he'd like people to be taken aback, not knowing who he was. Simon fits better, because as far as we know, he wasn't as heavily requested as the others, but nonetheless iconic. But I never said exclusively fan favorites. But Ridley, K Rool, Banjo, and Geno would be bringing back four heavily requested characters since Melee, which makes better sense for the Ultimate Smash game for fans.
As for new stuff--this is extremely subjective, but you could do a lot more with Banjo & Kazooie from what they do in-game alone. Yes, you could make up stuff that Steve doesn't do in game--Sakurai has shown he can make any character, if he wants to. For me, it's a question of should, rather than could. And looking at Duck Hunt, it looks like Smash could have drawn inspiration from B&K--which is high compliments, if true. But there's a wealth of moves that can be drawn from both games that could make B&K one of the more unique characters on the roster (
you'd never see the Dog bash people over the head with the Duck), especially since it's a game all about learning new and crazy
moves as you progress. Banjo & Kazooie, to me at least, has the unique moves and the character, making an overall better package.