Each forum (sub) will run its own instance of the code. The code is open source, so anyone can modify the code and run that. This design means that each forum must have its own domain. So rather than www.freedit.org/r/lizards it would just be lizards.freedit.org/ .
Originally I wanted to implement modules and views for built-in flexibility. Now I changed my mind. Modern popular software is totally inflexible and I am not going to count on modern programmers writing modules. So instead the code will be brutally simple and directly implement the functionality. Options will be handled with simple if-statements. But configuration will be done by entering Luan source into a textarea. This code will be run at site initialization. And this code can basically do anything. If anyone wants some special configuration, I can just expose what needs to be changed in the base code, and then the configuration code can change it as needed. So this approach maximizes simplicity and flexibility with the only down-side being no clean API for modules.
Actually this is where I started, and this has been implemented.