I already did a massive blog post about what my ideal roster for a Mario spin-off was, but after Super Mario Odyssey, some updates should be noted. I'm not going to rewrite the whole thing again, instead, I'm just going to do a basic outline which hopefully will be self-explanatory. With that in mind...
Since they already have models for them from Ultra Smash
Characters who SHOULD be obvious inclusions but might get passed over for who-knows-why
Characters who will probably be included even though I'd hate it if they were
Baby Mario (well, I guess I wouldn't mind as much if it were only him)
Pink Gold Peach
Other characters I wouldn't mind seeing
Petey Piranha (doubt it since he's a boss, but still possible)
One of the Broodals (most likely Topper and maybe Hariet)
Larry Koopa (if any Koopaling makes it in, I hope it's him)
King K. Rool
Characters who definitely won't be playable in this game but I'd want to see anyway
Donkey Kong Jr.
Blue Bowser (Bowser's brother from Super Mario Bros.: The Lost Levels; I like to call him Bluser)
All Seven Koopalings (one or two of them, maybe, but all seven, highly unlikely)
Professor E. Gadd
Ashley (from WarioWare)
Stanley the Bugman
The Kings from Super Mario Bros. 3
Mad Scienstein/Dr. Arewo-Shitain
...And that about covers it I think. Here's hope Nintendo surprises us some more with this game (and GOOD surprises at that).