I would argue that Jade is the Witch of the East

The witch of the east was the one who originally wore the ruby slippers, and Jade is coming from the right side (East) traveling left (West).

Outside of that though, I want to say that John is Dorothy. Because while they aren’t really the “leaders” of the group, they are still the glue that holds the group tightly together.

I want to say that Dave would be the Lion because he is trying to gain confidence and courage to be as cool and brave as a real hero, like Bro and John.

Rose would be scarecrow because Dave would put her there for ironic purposes (she has all the brains being in the brainless of the bunch). But Scarecrow wasn’t dumb, just ignorant of the world because he was stuck to a pole all his life. So they are both trying to learn more about the world they are in, and use that knowledge to help everyone.

And Jack would be the Wicked Witch of the West. That one should be obvious.