Responsive admin template
Please implement responsive design on the admin template. Most of the time I use whmcs from my laptop, but sometimes I need to give a quick update to a support ticket or manually unsuspend an account, comment on a project etc. These simple activities are very time consuming when the backend is not mobile friendly.
While you're at it , it may be worth thowing in a progressive web app manifest file that will allow for adding icon to the home screen in mobile browsers, and displaying in full screen .