Support for RESTful APIs in dynamic DNS, in particular for custom DNS providers would greatly expand it's usability.
Support is already mostly there with the dedicated endpoints such as aws and gandi, but adding your own requires going into the CLI and copying and adjusting one of the existing endpoints in /usr/local/opnsense/scripts/ddclient/lib/account/.
The ability to set headers and content in the GUI for custom endpoints would really expand it's usability.
The interval of dynamic DNS lookup should also move to cron instead of being built-in, perhaps even per-account, so it's more in line with other services such as ACME client.