Hey Alessandro. I think it would be cool to show your approach to work with the comments section of WordPress. I find it pretty hard to customise that section according to the respective needs of a website. WordPress only offers the three basic fields, what if I need a custom field. How would you implement a Google ReCaptcha etc.
For the custom fields I didn't find a good plugin, for the ReCaptcha, however, there is one but same as you don't like plugins, so do I.
Another great thing could be the management of user roles and, maybe, even the generation of new capabilities. There are also numerous plugins to do so, but they all offer different things, so at the end, one ends up using two of them, which I really don't like as it generates a lot of mess in the backend. Having an own plugin to manage those capabilities one cares about would be a very neat thing.
Anyway, thanks a lot for the superb series, I did learn a bunch of things and enjoy every new lesson. Keep up the great work.
PS: Did you code this awesome forum yourself? It really is mind-blowing!