I thought somebody would've found this out by now, but apparently, NintendoNow.com said on March 31 who was going to be in Mario Kart 128. I can't remember all of them, because there were thirty (yes, you can squeal with joy now), but I remember you could be:
the regular 8 characters,
Dr. Mario,
Baby Mario,
Baby Bowser,
E.Gadd,
King Boo,
Big Boo,
Goomba,
Diddy Kong,
Dixie Kong,
and all seven Koopa Kids.
Although, on April 1st, the site pulled a lot of April Fool's Day pranks, so this could be one of them. But, I know they provided a source for this update, so someone should check it out.