FastMail has been working on something called JMAP [1] for quite some time. It's an HTTP-based replacement for IMAP. Perhaps it could be extended to replace SMTP as well. Then we would have a single, HTTP-based API for all of our email needs.
I'm not convinced that's a useful generalization. Beyond that IMAP and SMTP do something vaguely related to email, there's very little overlap between the two protocols.
[1] http://jmap.io/