I have done all types of customization in Whmcs , provisioning modules, payment gateways, registrar modules, order forms , templates , addons, reports , hooks .
Some of my work so far includes the following:
Provisioning modules:
Whmcs module using proxmox api
Whmcs module using 4psa API
Whmcs module using vsphere ESXI
Whmcs module using interspire API
Whmcs module using aql API
Whmcs module using connexient API
Whmcs module using axess api
Provisioning module using sawmill API commands
WHMCS Provisioning module for Cloudflare
WHMCS Provisioning module for Cloudstack
WHMCS Provisioning module for Xen hipervisor module for VM creation
Updated Parallel Plesk module as per client specifications
Besides this I have integrated custom solutions with whmcs using whmcs external API .