I have 2 suggestions for the Proxmox VPS For WHMCS Module:
1. Connection to the DNS Manager
Since your DNS Manager supports rDNS, I think it would be a greate feature if you have a connection between the Proxmox Modules and the DNS Manger. So that the VPS Client can set his rDNS from the Proxmox Clientarea himself. It relieve the support and the Client must not wait to the support makes the entry, which is great for the client.
2. Automatic rate Limit
Since Proxmox supports the Rate Limit for LXC and KVM, it would be great if you supports traffic Limits. What I mean is: The most providers give the client X GB Full Speed and when the Client reaches this limit the provider give you a lower Rate Limit for the rest of the month and reset the Limit in the new month. It would be nice if the module can do this automatically.