Imagine adding label sendlater/tomorrow/11am to a draft, to have it automatically sent at that time. It would be relatively straight-forward within the current framework. Unfortunately, Google apps script doesn't support sending drafts. People have suggested various workarounds:

• https://gist.github.com/mogsdad/6515581#file-sendgmaildraft-js does it through the more direct Google API, as far as I know not through GMailApp. Could be worth looking into.