What good is 20 GB if it's limited to message attachments? Most of what I've seen discussed in this context is not insufficient storage, but loss of features, specifically files and photos and calendaring.
If there's going to be an intermediate between Basic and Premium, I think it should remain at 1 GB of storage but reinstate the other features that were removed on August 24. It's already being supported in legacy Basic anyway...most of the additional coding would be limited to shoehorning it into the pricing structure.
Other options strike me as more long-term and unlikely to be implemented before December 15.