Based on my observations of this Smash game, I think I'm onto at least something with my most recent predictions for newcomers. We're having less than even the smallest amount of newcomers from before (14, what Melee had) with base roster and meeting that number with DLC. This title also seems to be based on mostly fanservice, so I'm taking that into consideration here.
Base Roster Newcomers
1. King K. Rool - Sakurai has thrice now acknowledged and added a character who's popularity is primarily in the West, first with Diddy, then Little Mac, then Ridley. Knowing that at least amongst the online fanbase, K. Rool is popular in both East and West, and since there's never been any ill-will towards Rare created characters (Diddy and the Krystal assist trophy prove this), I think the King's time is finally here.
2. Ashley - With his insider information, Sakurai would likely know of the upcoming WarioWare: Gold and could theoretically have prepared a newcomer for promotional purposes; beyond this Ashley is incredibly popular in Japan and has notable popularity in America and Europe as well. Her assist trophy is currently missing from the roster, whereas other frontrunners and popular picks like Takamaru, Waluigi, and Bomberman have all been shown off.
3. Generation 7 Pokémon - Generation 7 is an incredibly important game to the Pokémon franchise, being the celebration of the 20th Anniversary of the franchise and providing a number of much-needed tweaks to the standard franchise formula; furthermore as with most Pokémon games sold like hotcakes. It was also heavily promoted along with a handful of 'mon in the game. While I don't personally see Mimikyu or Lycanroc likely, I could see either Incineroar or Decidueye making it playable (especially since we no longer are missing part of the type trio.)
4. Dillon - While this game is a bit more obscure, I can see Dillon chosen on uniqueness alone, representing either tower defense mechanics or cowboy tropes - as he connects to both themes. Furthermore, he had an upcoming game during development which as we've established Sakurai could have known about. He's also on Sakurai's radar as he was an assist trophy in the last game, and, like Ashley, is currently unseen in Smash Ultimate.
5. Spring Man - I wouldn't say he'd make it in if it wasn't for the fact this game's development is so different from past Smash titles. Veterans, despite having to be remade from the ground up, still take less time to make as they have an established design and movesets already created, and Sakurai has already stated not to expect too many newcomers; this combined with the fact we see an ARMS item appear in Smash Ultimate makes me think he could have been added on to the roster plan mid-development when ARMS was at the height of it's popularity, and as a fighting game itself it's easy enough to translate into Smash.
6. Dixie Kong [Possibly as an Echo Fighter/Semiclone of Diddy] - Dixie's relevance is ensured by her recent playable appearance in Donkey Kong: Tropical Freeze and the fact she shares a very similar body to Diddy means she could make for an easy clone or semiclone, or even a new "Echo Fighter" of Diddy. Sakurai's shown to take creative liberties when developing semiclones or echo fighters (such as Daisy), so it's possible he could do so with Dixie.
7. Impa [Sheik's Echo Fighter] - Sakurai's giving the Zelda cast a huge set of visual changes this time around, with each one representing a different title in the series, and being an actual Sheikah, seeing Impa utilizing the moves Sheik uses would not be a stretch. I think they'd likely go with her Skyward Sword design, as that most resembles Sheik's body type and is a currently unrepresented title in Ultimate.
8. Octoling [Inkling's Echo Fighter] - With the popularity of the Splatoon franchise being so massive, I don't think it's out there to see two playable characters enter the fray in the same game. Beyond this, Inklings and Octolings function exactly the same in their own series - making for a literal perfect Echo Fighter. Beyond that, they're popular amongst the Splatoon fanbase, are an additional villain representative (sort of?) and had the upcoming Octo Expansion in Splatoon 2 which Sakurai could have known about.
9. Simon Belmont/Rayman - One of these two third party franchises is likely to get represented; Castlevania is returning to relevance with cameos and the anime series, with rumored titles in the franchise to soon make an appearance. Konami is also fine with representation in Smash as Snake is returning and Bomberman, Simon's only true competition in-company is an assist trophy. On the other hand, Ubisoft's Rayman is already on Sakurai's radar as his franchise received not one, not two, but three trophies in Smash on Wii U, including Rayman himself. We've also already debunked the "no Western characters" argument and though he does like to meet with the characters' creator, Sakurai does go back on his traditions and words sometimes (see Ridley, Pac-Man, and Villager as good examples.)
DLC Newcomers
10. Rex & Pyra - Sakurai has stated on multiple occassions that he loves Xenoblade Chronicles 2, and the gimmick of Blade switching is a very easily translatable one. The franchise has been represented in Smash with Shulk, and despite releasing last year, it was Nintendo's holiday game and is still being supported.
11. Edelgard - Or any main character from Fire Emblem: Three Houses. I personally think Edelgard is most likely due to already being the fan favorite and the only one we know so far capable of wielding multiple weapon types. In addition; Three Houses comes out very soon after Smash and would be ripe for promotion. While Sakurai does think the franchise might be overrepresented, he didn't take anyone out and his team could easily convince him of another fighter's uniqueness.
12. Generation 8 Pokémon - This all depends on how soon we get Gen 8, but it's allegedly coming out next year and I think Sakurai might be supporting Smash with additional DLC for quite some time. Who it is, again depends on marketing, but it'll likely be a starter and the type not represented by the Gen 7 'mon (assuming it's Incineroar or Decidueye).
13. Simon Belmont/Rayman - Whomever doesn't get in the base game.
14. One Last 3rd Party - I don't know who, but something to try and top Cloud and Snake, someone big, popular, and guaranteed to sell. Sora I could actually see being this one.