It would be really helpful to have a nickname box, particularly when it comes to sending email templates. All too often a person is known by a nickname or abbreviated name so sending an email to Dear Stephen sounds very odd when I know the person as Steve. If there is no name in the box, it could default to the first name, but if there were a name in the box, it would be handy to have the option.
I just wanted to give an update that I think we are part way there on this request.
You can now add custom fields to the Organization & User Form. Therefore, if you reach out to support you can have them add Nickname as a field you can capture.
That said the piece of this being an email merge field is still on the roadmap so I will keep you updated as we continue to roll out functionality around the organization and user custom fields.