Please hack on the Template Toolkit. It has been designed to be extensible and has literally dozens of programmer hooks that you can tap into to do all sorts of cool things. The object oriented architecture allows you to create your own subclassed modules to implement your own services, providers, filters, plugins, and so on.
The Template::Manual::Internals document gives a brief overview of the architecture. Unfortunately, it's not as complete as it could be, but the code is well documented and generally easy to follow. Don't be afraid to use the source, Luke.
The internals document also contains information about preparing and applying patches, updating the documentation and various other useful tips.
The source code repository for the Template Toolkit is hosted at Github.
https:/abw/Template2