okay im gonna put my predictions here just in case i somehow do manage to get something right
10 new characters, 4 vets and 6 newcomers
Vets: Ice Climbers, Wolf, Squirtle, Ivysaur
Newcomers: Inkling, Ridley, Captain Toad, Chorus Men, Dixie Kong, Chibi Robo
I know it's bold to only predict 10 characters and no 3rd party but the roster is HUGE, and characters aren't an addition; they're a multiplication. And if they're only adding 10 characters then instead of the usual criteria (e.g. "we need a retro rep!!!!", "where's our WTF char???",, "3rd party or bust!") I think Sakurai will try and round off the roster with popular requests (bringing back Wolf, putting in Ridley), putting in missing universes (Inkling, who also was a really popular request, Chorus Men, Chibi-Robo), and putting in the missing vets (Climbers, Squirt/Ivysaur) and all-stars (Toad and Dixie).
Why Squirtle and Ivysaur? Gen 7 doesn't have a clear frontrunner and Pokemon isn't in dire need of a new character... with Greninja, their jump to the 3DS and its generations are represented. Why does Alola need a new character, just because it sold and it's new? Since if that's all it takes to get a character then this roster would look entirely different. With Squirtle and Ivysaur, they're unique veterans that would make sense to bring back if they're truly trying to round off the roster. Per Emily Rogers' leak about the roster being "Ultimate"; how can they claim to have an ultimate roster if they still have messes to clean up? It's like not doing your work in school then asking for extra credit. It doesn't work out.
If I added three more characters to this prediction though I'd put Rayman, Takamaru and Bandana Dee. A fourth would be Isabelle.