The WHMCS module is very basic and lacks a lot of features. Some features to integrate and consider are to integrate WHMCS social/2auth integration with Plesk social login/2auth. Another integration is to allow the client to control their installation from the WHMCS panel and add configurable options such as plugins. Also, the client shouldn't be required to set a domain when purchasing a service, it should only ask for hostname since some will have multiple domains on their Plesk.