Nico Minoru: Brenda Song(Not a perfect match, but just imagine her in goth gear)
Karolina Dean: Evan Rachel Wood
Alex Wilder: Lee Thompson Young(With Fake fro)
Chase Stein: Taylor Kitsch
Gertrude Yorkes: Unknown
Molly Hayes: Unknown
I really think that those last two characters should be found through open casting, because you really need something special for them.
Oh, and of course, for Geoffrey Wilder....