I celebrate the proposal! We only used core reporting once quite some time ago and we moved to HTML reports about 5 years ago and it is much better!
In the html_report module we make heavy use of jinja macros (which users can modify from the Tryton interface) so there’s no need to modify the whole report just to change how a particular section such as the lines of the sale or the customer address.
I’d suggest providing some kind of inheritance or customization that does not rely on completely changing the report.