Some suggestions for visual (Favicon, Custom CSS, Admin Login BG)
Simple suggestions that can be quite useful.
1. In the administration panel, add an option for favicon.
1-1. Add favicon on every page.
Example of a page that has no favicon: whmcs/templates/default/viewquote.tpl
Among others I do not remember at the moment.
2. Add a new CSS file to templates (eg custom.css) to override the default rules.
Thinking about the WHMCS customer who does not want a custom template, just some
simple CSS changes.
If you change the original CSS file, future WHMCS
updates can remove these customizations.
3. In the e-mail templates, add variable to "view this email directly in the browser". (https://requests.whmcs.com/responses/email-content-view-in-browser-link)
4. Option to change the color and background image of the login page (administration).