I don't understand your use case or the proposed workflow.
There are already now ways to include things from other files into the firewall config as e.g. miniupnpd and bcp38 do already now. Both use "include" config option. Bcp38 reads config from an uci config file and generates the iptables rules for firewall.
I you suggest that there should be a new "pre-uci" rule format to be converted into uci and then parsed to ip(6)tables rules in firewall config, I don't see devs really warming to the idea.
the format will be uci but instead of directly run iptables script will generate uci config that are included in the main firewall file. The main advantage is the portability and the fact that luci could parse this kind of file and offer a read only view.