Maintaining additional domain fields is unnecessary complex in the current way of maintaining it over config file.
How to improve:
(1) Having a possibility for registrars to get this integrated over a registrar module method. Some nomenclature like " <registrar>_AdditionalDomainFields". That would allow a registrar to return the necessary fields instead of enforcing manual maintaining such a configuration file
(2) Right now, it is impossible to have additional domain fields separated for registration and transfer which is for some TLDs the case. That registrar method could then get the order type forwarded like "registration" and "transfer".
(3) Whenever a new TLD got added by a registrar requiring additional domain fields or for a TLD the additional domain fields configuration has changed in registrar API, manual effort is necessary to align the configuration file in WHMCS. So a client has probably first to run into an order issue and to ask his reseller for help - which is bad user experience in WHMCS.
That described new registrar module method is IMO the best way to decrease module updates and configuration efforts and thus helping resellers a lot and also registrars who have a possibility over their API to get additional fields returned. Registrars who have not this possibility can return their static configuration there too and would also benefit of the possibility to distinguish between registration and transfer processes there.
This feature will make a lot of people lucky. The most support request that reached us in the past were about problems with additional domain fields and this would fix all that in ease!
I am in hope that you guys like that feature request and it will reach WHMCS 8 soon :-)