I think the powerup assignments take into account the availability of shields, like whether or not they're used in levels already. If the assignments were changed, you'd never get the fire flower in the vanilla levels, for example.
Maybe the different shields could be an idea for a suggestion?
I checked the models to make sure it wasn't already done, and only the lightning shield currently has a model.
Just trying to stay with more easy, simple things.
at least, compared to characters.