I think what would be helpful is some mechanism that allows template authors to auto-generate tests. For example, if i could run something like:

nuclei -t mytemplate.yaml -u <vulnerabletarget> --autogen-testcases 

And have it spit out HTTP request and response data that can be used to validate the templates without a full vulnerable target, that would be awesome. Template authors could effectively start writing basic mock tests using a single command.

I think the system, for http templates at least, would need some way of hosting a test web server which then used the generated response-data from the known vulnerable service. This would be very basic and wont fully test more complex templates, but it would be a starting point.