It is incredibly handy to have the ability to compose and effectively send an email, but to have the option to schedule the date/time for the message to actually be sent out. It could be an additional button next to the Send, Save Draft, etc button on the top. Thanks!
There is a plugin called scheduled_sending from the MyRoundCube project (commercial) http://myroundcube.com/ that allows delayed email sending. This isn't a feature that likely will make in into the core of RoundCube as it requires additional setup.