I think the 4 last characters will be

,

and 2 newcomers, since they usually add 2 newcomers in the main series lately (MP 7, 8, and 10 all added 2 new characters, 9 added 3 new).
In handheld MP games, there are rarely any newcomers (only

in IT), but this game needs a lot of playable characters because of Toad Scramble.
So for the new playable characters, personally,

and Luma would be nice. I guess Baby Mario is a possibility, although he usually appears with another baby.
Knowing Nintendo, I wouldn't be surprised if the newcomers are a Sprixie Princess and a random enemy like Monty Mole lol.