My naive suggestion is to add a method like .subclass(className: string) to the block / item builder that looks up the class by its fully qualified name and checks that it's actually a subclass of the appropriate base class. It would be nice to, for instance, add custom "incomplete" items for Create's sequenced assembly recipes, which need to be an instance of SequencedAssemblyItem to properly show the progress bar on the item icon. I think allowing arbitrary subclasses is a nice general solution that works for other use cases too.

Support for item types has been added in the latest version (including the sequenced assembly item in the newest KubeJS Create), block types will probably be done at another point, but should be easy enough as well ^^

Y'all are awesome. How do I use it? I can't find anything in the docs and a cursory glance at the source code hasn't turned up anything obvious yet. Whoops, I forgot KubeJS-Create is a thing. Found it now!