To further boost the flexibility of WHMCS it would be good to have a customer facing API. This would allow customers and often resellers to use the system of their choice to resell the services offered through WHMCS. The system they use doesn't necessarily have to be WHMCS although it would make sense to have some existing modules that can be given to customers to link up a different WHMCS installation to the customer account of the primary system.
I think that the customer API should be a feature that should be there anyway. The module(s) that link a different WHMCS installation to the primary system using that customer API can be a paid module as far as I'm concerned. A server and registrar module would be sufficient.
The way I see it this will both boost WHMCS' business as well as ours. The maths is easy.