The IETF is the wrong place to go about design, so here goes.
It seems to me that S/MIME already has the functionality of a dropbox scheme. What is lacking is what the usability folk call the 'affordance' of being used in a dropbox fashion and well, usability.
Further, for such a scheme to become widely used it will be better if the scheme is a protocol that can be supported by existing service providers as an extension to their current offerings rather than a completely new scheme that attempts to disintermediate them.
There are two separable forms of authentication required in such a scheme. First we have the the access controls to the remote mailbox, second we have the cryptographic controls on the data. My bits may b encrypted but that does not mean that I want just anyone to view them. And even encrypted bits typically leak some metadata.
* The recipient will need a public key against which senders can encrypt their data.
* Senders will require a mechanism that assures them they are using the correct key. If the parties know each other, a 'strong email address' would be sufficient, that is a fingerprint of the public key combined with the destination email address. If the parties do not know each other, a Trusted Third Party (or parties) will be required to provide an introduction.
* The mailbox provider can map the key identifier to the public key if necessary.