My clients are often confused with all the logins they must endure. WHMCS for accounting and support, their control panel (either cPanel or PLESK), FTP, emails...
If there was a way to login to the client's control panel from WHMCS this would be a fantastic feature. It would require a new database field for the URL of the control panel.
To make it easier, you could specify the control panel login/password has to match the control panel login/password so when they click on CONTROL PANEL it would automatically log them in.
Just my 2¢